I think a logia user having a subtle element like fire,magma,light,lightning etc
cannot use Busoshoku Haki along with their logia elements .Cause these elements
do not have a form or shape .I can't even imagine a Busoshoku haki imbued in fire
or lightning .
when they were in their elemntal form ,we saw how akaoji got hurt when kozu broke
his ice form ,he spit out blood .why did not he imbue his ice form with bushoshoki haki ?
cause it very difficult form them to control both the things at the same time .
It is the same case with Akainu too ,when he was attacked by marco who was also
confirmed to have BH haki .He did got hurt .Now if akainu had imbued his Bushoshoku
haki with his magma element .he could have pawned amrco.why did he not do that .
Take the same ase with Kizaru Vs Rayliegh .Kizaru would have gained an upper hand
over rayliegh only if he has imbued his light elemental with his BH haki but he was
having difficulty facing Rayliegh who did not have any DF ability .All these evidence
shows that an logia user when uses a subtle logia element like fire or light cannot imbue
it with Bushoshoku haki .
here is an evidance that shows that
kizaru was not using his bushoshoki haki when he was using his elemtal powers .
Marco was using bushoshoki haki and kizaru was using his elemntal power here .
Even here is an evidance that shows that kizaru was not using his bushoshoki haki
when he was using his elemtal powers .
Even if he assume that kizaru
did use his haki when he using his elemental
attacks over marco,then his haki was very weak because marco's kick
did send kizaru flying away.This is the same case with akaoji too ,
when kozu broke akoji's ice form (he was completely dependent on his
logia DF not on his bushoshoki haki) .If he did imbue his ice form with bushoshoki haki ,
then Oda would has definitelt given us a clue or a visual distinction like how he did
with luffy (elephant gun ) .
So when Akainu or Akoji fights Enel we have to see their elemtal superiority not haki .
Even if the admirals does posses haki ,
I don't think their haki would be strong enough to stop a bolt of lightning .