Just no.
You really think Jiraiya was going 100% against 4tailed Naruto? You think he was trying to kill him? No. He was holding back and trying to play it safe so he didn't hurt him.
Kisame doesn't stand a chance against Jiraiya. Kisame's biggest advantage against anybody is his large chakra reserves. This is meaningless against Jiraiya because he also has immense chakra reserves.
Kisame's second advantage is his ability to absorb chakra with Samehada. Jiraiya would quickly learn this and avoid using chakra based attacks such as the Rasengan, fire techniques, etc. To top it off, once Jiraiya goes into Sage Mode Samahada can't do anything. If it absorbs Sage chakra it'd turn to stone.
All Jiraiya has to do is use Sage Mode and a few summons. Kisame, even fused with Samehada, wouldn't be able to match SM Jiraiya's brute strength.
Even Kisame's Water techniques wouldn't be helpful. Jiraiya can use Earth, which is strong against Water.
First off let me say this, EVERYTHING you said is completely WRONG.
One, samehada would never turn to stone from absorbing sage chakra, samehada is a chakra CONVERTER. Why?
Kisame said that he and samehada have the same chakra and thus it should be impossible for anyone to sense his presence inside samehada. Tell me, how do you think that happened? That their chakra signitures became the same? we all know that Kisame's is not the first wielder of samehada, the wielder before Kisame was his sensei, obviously Kisame's chakra was not the same as samehada BEFORE he became its wielder, the only way it could have become the same is if samehada CONVERTED its chakra to be the same as Kisame's when he became the wielder.
Two, we know that BIJUU chakra is insanely corrosive, look what happened to orochimaru when he made contact with four tailed naruto, he completely corroded and had to spit out new bodies numerous because of the sheer corrosiveness of the chakra. Naruto, a JINCHUURIKI, had his skin completely corroded and sakura had to heal him, so how did samehada steal SIX tails worth of youki and give it to kisame without BOTH of them getting corroded or having any side effects? I'll tell you why, because samehada is a CHAKRA CONVERTER.
Three, even the Enka Guy, Master Kin I think the name is confirmed it, he analyzed that samehada was absorbing the eight tails chakra and giving it to kisame IN THE FORM of PHYSICAL ENERGY, that was what he said word for word, the phrase IN THE FORM directly implies that the chakra was NOT in the same state as it was before, another testimony to samehada's chakra CONVERTING abilities.
Three situations that attest to samehada's chakra converting abilities, one of which is a direct testimony by the Enka Guy.
SAGE CHAKRA is rendered USELESS to samehada, if it can convert Youki than there is no reason why it wouldn't be able to convert sage chakra, which is far less potent then Youki, Samehada WON'T turn into stone from absorbing Jiraiya's chakra.
Now you said that Kisame's water jutsu would be useless against Jiraiya's earth Jutsu because earth trumps water, that is incredibly naïve of you, kisame has an ultimate water technique that renders any ninjutsu, earth or not useless against him, the great water shark technique that kisame used against Guy was described as invincible by Kisame himself because it ABSORBS ALL THE CHAKRA it comes into contact with and uses that chakra to become BIGGER and STRONGER, giving the jutsu a potential LIMITLESS GROWTH, Jiraiya's earth jutsu will be useless because the is no jutsu without chakra, the chakra used for any earth jutsus Jiraiya has will simply be donated to the great shark jutsu and make it bigger and stronger, killing Jiraiya in the process, the only reason Guy's afternoon tiger worked was because it was a jutsu that was SUPER TAIJUTSU, the agfternoon tiger does NOT have CHAKRA. Besides there is still samehada too to deal with, assuming samehada doesn't betray Kisame like he did against Guy, which was bullshit excuse for Kishi to have Kisame killed, plot no jutsu I guess.
Great Shark Jutsu and Samehada make all of Jiraiya's ninjutsu useless, swamp of the underworld included, samehada simply swipes that chakra, plus I'm pretty sure that samehada can break kisame out of frog song, though I'm speculating on this, but if you have been paying attention you'll understand why it should be plausible.
SUMMONS...
I don't know why people think that toads can trump SHARKS in a WATER BATTLE, seriously, you guys think that Jiraiya's toad summons will beat Kisame's SHARK SUMMONS on the WATER, no way man, that is blasphemy, this isn't even up for debate.
Jiraiya chakra reserves, sage mode or not are like comparing a lake and an ocean, compared to Kisame, he has a lot of chakra by ninja standards, but the gap between him and Kisame is like the distannce between heaven and earth in this category, no comparison.
Kisame can create like lakes or mini oceans, it is irrelevant whether they are near a lake or not, Kisame trumps Jiraiya 10/10. Its like Kisame said, the stronger his opponent, the stronger he becomes.
Your excuse is that Jiraiya wasn't trying to kill 4 tailed naruto?
Right, so Kisame was totally trying to kill Bee right? I mean it aint like he had to bring him in alive right? No because akatsuki totally kill their jinchuuriki targets yeah?
Oh and did you say that Kisame will lose to Jiraiya's brute force?
Ahem right sure man, that is possible, I mean it aint like Human Path, the weakest of the pains completely stopped Jiraiya's taijutsu attack with his BARE HAND while he WASN'T EVEN FACING JIRAIYA right? Because that totally didn't happen right?
Oh and kisame is physically weak right? It aint like he wields a sword that can grow up to TWICE his own size right? It aint like a mere swing from his and suigetsu's attacks caused craters to form on the ground when they were sparing right, it aint like he broke himself from the his bindings after taking a direct hit from 7 Gated Guy's afternoon tiger and a super punch to the torso after the afternoon tiger from might Guy, then after breaking those bindings, which in itself was a strength and endurance feat that completely shocked Guy,Yamoto and Naruto, and he still had enough strength and energy to capture them all in his water prisons, summon a shark to escape with the info, and then summon five more sharks to commit suicide.
Afernoon tiger, seven gate guy punch to the torso, break super strong wood element binding, and still have energy to capture all of them with water prison and shark summons, all. Done in succession without samehada. Also, wielding a weapon that can grow to twice his size in battle and causing creators with mere swings of his sword.
Yes, I can see how the same SAGE MODE Jiraiya who had his surprise taijutsu attack EFFORTESSLY stopped by one of pains weaker path, human path, can easily overwhelm Kisame with his 'unstopable' brute force.
And yes, I can definately see how the same sage mode Jiraiya who had his arm RIPPED off by asura path can overwhelm the Kisame that too afternoon tiger head followed by 7 gate punch to the torso head on can be overwhelmed by this guy, the guy who was overwhelmed by the same asura path that was one shotted by Akimichi chouza's punch.
Yes the same sage mode Jiraiya who had his throat effotessly crushed by asura after having ripped off his arm will overwhelm kisame with brute force, totally believable dawg, totally believable. (Insert sarcasm)