"Regular" mollies are smaller than sailfins. Balloon mollies are "regular" mollies that have been bred to have a very large round belly and a shorter body. This means that they tend to have digestive problems because of the contorted nature of their organs due to the severe shortening and "fattening" of their abdomen area. I would pick "regular" mollies over balloons any day.
I would not say that one is more aggressive than the other. Mollies are quite peaceful. They are curious and might try to eat things like snails or shrimp just because they are hungry little beggars. They seem to be the most puppy-like of the fish I've encountered. They are always begging at the front of the tank whenever they see me.
I tend to think they do best with very light addition of marine salt as my most healthy ones are in my brackish tank. I've had juvies/adults survive in my 75g freshwater community, but they seemed to grow slower and not be quite as healthy as the ones in my brackish (1.008) tank. I don't think you need to go all the way to brackish, just add a small amount of marine salt. If you go that route, mix up your water in a separate bucket (rubbermaid tubs work great) the night before a water change and make sure that all the salt is completely dissolved. Powerheads work great to make sure the salt gets worked in.
I would not say that one is more aggressive than the other. Mollies are quite peaceful. They are curious and might try to eat things like snails or shrimp just because they are hungry little beggars. They seem to be the most puppy-like of the fish I've encountered. They are always begging at the front of the tank whenever they see me.
I tend to think they do best with very light addition of marine salt as my most healthy ones are in my brackish tank. I've had juvies/adults survive in my 75g freshwater community, but they seemed to grow slower and not be quite as healthy as the ones in my brackish (1.008) tank. I don't think you need to go all the way to brackish, just add a small amount of marine salt. If you go that route, mix up your water in a separate bucket (rubbermaid tubs work great) the night before a water change and make sure that all the salt is completely dissolved. Powerheads work great to make sure the salt gets worked in.