Fire + Water: I believe the water, depending on the rank difference, will cause the fire to fizzle and burn out. More water less fire.
Alright so in our RP, water is stronger then fire. This being said, a C ranked water jutsu is equivalent in power as a B ranked fire jutsu, keeping in mind that sizes of jutsus are approximately the same. This is like because water is capable of extinguishing flames and thus not letting them keep on burning. Although water is stronger then fire, when used in comjuction the two are capable of forming steam. Although boil release can only be used by Mei, the steam can be formed once fire and water clash in a battle. This is especially usable if you are a sensory ninja or are capable to in any way or form know where your opponent is standing. You will be capable of hiding your presence and thus create sneak attacks. Now if you know that your opponent is capable of sensing you could get rid of the steam by releasing wind which will passively blot away the steam as it continues towards opponent.
Fire + Wind: I've been told that the combinations of fire and wind produce bigger fire if the chakra levels of both techniques match.
Now when it comes to fire release and wind release. Fire is stronger then wind, because oxygen from which wind is formed is just going to fuel the flames. Although when they clash fire is going to overpower wind, they are actually quite great when it comes to their combo usage. Oxygen from which wind is formed is going tu fuel flames even more and thus make them stronger and make them effect bigger area. The speed of the technique that would now be running would be greater.
Fire + Earth: I believe Earth would be capable of blocking certain katon jutsu. Maybe even put it out if the user does something to contain the fire, sealing out the oxygen perhaps.
Fire and Earth Release in our RP are neutral with each other. Which means that a C ranked fire technique will be equivalent in power as a C ranked earth technique, keeping in mind that their size is approximately the same. Although they are neutral to each other when they clash they are quite good when they combine with each other. The heatness from fire will effect the earth and thus melt it forming magma. Keep in mind that magma will be formed if earth is more solid. If the variation of earth release, mud was combined with fire release, heatness of fire is going to harden the mud.
Fire + Lightning: This I have no idea of how ever...I've never seen or encountered it.
Same as with Earth Release, Fire and Lighting are neutral once they clash. It is usally great to make great damage to opponent. Fire release is going to burn down the opponent whereas lighting is going to shock opponent. Interesting to the two is that they in a way compensate each other. Fire is strong against Wind whereas, Lighting is weak against Wind. It would be good to let's say firstly send a lighting attack as a bait, assuming that opponent is going to use wind jutsu you send a fire attack to follow it.
Katon is mainly used for explosive jutsu that end off in a heavy heated impact or being covered in flames. It has a wide range and can be compacted into fireballs and at some points animalistic shapes are crafted into jutsu. Heat being one key element of fire. Heat alone can harm someone. The impact of being hit by something that engulfs you in fire. Capable of melting, burning and other relatively dangerous attributes.
Katon or fire release is popular for its offensive power. Its power to burn as well as its violence. It's the third fastest element. First being lighting and second being wind release. Techniues of fire release are formed by using nature manipulation on chakra and changing properties of chakra to the one of heat. Fire release techniques are usually released from ones mouth although can be released from other parts of the body. Fire can also be infused into ones sword and thus make it hotter and thus make it's power greater. Now when it comes to fire release, there are few variation like flammable gas and ash.
Now if you don't have any questions we can proceed.