Don't worry the yo-yo's will get the hang of snail eating. It took mine alittle time and once the biggest one learned the 4 others caught on from him. As for adding more molly's, I don't know how old or big your angels are but they will get big, and if you have a pair or two you might have problems already being the tank is a 55g. I would add no more than 2 more and I'm really not sure I'd do that because of the angelfish issue. If you were to move 2 angelfish when they mature into another tank then having 2 angels and 3 mollies wouldn't be a big deal. Just remember angelfish grow and so will the mollies. I used to have sailfin mollies years ago and those got to be some "big" mollies!
BTW, looks like you have a real nice planted tank from what I can see!