coby and Kaku

My question is: can orb boosting buffs stack?

no i dont think so. though if you wanna be sure  try it on 0 stam isle

kaku havent droped yet for me

I tried it, after using Kakus special, I wasn't able to use Cobys special anymore (for this two turns...) so the answer is no

And I hope I hadn't understand the question wrong, otherwise excuse 

that was my question. but the question will be actual again when more buffs come via new Charakters. i hope some of them will stack like coby and mr two

which mr 2...?? 
the normal mr 2 don't boost anything (just food to PSY orbs...)
the impel down mr 2 boost the attack from fighter, not the orbs, so they stack but they also don't have both the same ability...

coby and dial usopp for example DO stack. I think specials which do the exact same cant be stacked. Coby does 1.5x for 2 turns and dial usopp does 2x for psy. Makes me wonder though if doffy comes out can all 3 specials be stacked.. for some crazy burst?

I would say, you can use each type one times

So Dial Usopp give 2x ATK for some type of units (PSY) and Urouge who give also 1.5x ATK for some type of units (Striker)
These two specials can't get stacked together (as far as I know)

Coby is a different type of boost, he boost the orbs (no special type)

I think, thats why this two (Usopp + Coby) can stack
If I'm right, Doffy is also a unit, that boost the orbs, so you can use him with Usopp but not with Coby (for example)
But I'm not sure with all of this, just my idea