Yes we don't really know if Adam joined them or not. We have to wait for confirmation. We can only guess and right now my guess is that he is going separate way based on these two final scenes at the end of V5 and Adam's short : if Adam was willing to join Hazel and co, why show him in a tree watching them, remaining silent and waiting for them to pass before moving himself ? Very suspicious to me, I really interpret it as someone not willing to join the party. And it seemed to me that the final scene at the end of the short clearly happened after the "tree" scene. Adam was obviously alone and back on the ground but more importantly he did not move and behave like someone trying to flee like he did in front of Blake and Sun or trying to avoid his former "teammates" (my interpretation here). Also he still had his mask in the tree. So based on this assumed chronology, I think Adam is definitely going his own way (for now).
But we agree his chances of a solo run are low and that Adam is aware of this. Eventually, it is likely he will be looking to join Salem's crew but here too this needs confirmation. Also, I really want to know how Salem feels now about Adam. Yes he is a broken character but she sould make sure he is loyal because his latest actions don't speak in his favor (killing Sienna, trying to blow up Haven even with the White Fang still here). He looks like someone ready to disobey as soon as he feels something goes against his interests which is why I was asking the question if Adam is still willing to fight for the Faunus. If yes, then I wouldn't trust him if I were Salem.
Also very quickly, my intuition for V6 when it comes to the vilains is that Adam, Tyrian and Neo (yes Neo !!) will be the major factors. We will see a bit of Hazel, Merc, Emerald and Salem together and maybe something with Watts (who could be sent to Atlas if he is not there already). And I don't think we will hear from Cinder at all.