Yes, clowns should be added as a pair.
I forgot that you were doing the small tank first.
You could get away with another small fish such as a clown goby, small blenny (such as a tail spot) or a smaller goby (green banded or yellow watchmen would be good choices). If you choose to just do the two clowns, which is also fine, add 1/3 CUC before you add them, 1/3 with them and then test and all the last of them.
Your CUC should not starve in one week before adding the clowns to the tank.
As for your small hitch hiker snail/slug. It's probably a stomatella.