This is basically a downgrade of stack inserter, because it cannot place down stacks on belts as stack inserter does. Would it be possible to either add stack uranium inserter or just allow it to stack?

It is possible, but i don't want to.
It isn't a downgrade though, since if nothing else: the uranium inserter doesn't require electric power

Yeah, the power isnt really an issue at that stage in the game. I struggle to think of a situation where it would be great to use it though. Anyway, its you mod :D