Mollys fighting or playing?
 

I have 2 mollys and today I've seen that they seem to dart after each other, they never attack but one will randomly chase the other and then leave off.

They're both males, are they fighting/showing dominance or just playing?

Are the mollys male/female, 2 males or 2 females? If it is a male chasing a female than it is probably the male trying to mate with the female, if it is two males than it is probably the dominate male defending his territory.

two males, just been watching them and they're happily swimming along together. I'll keep watching them.

I have two male platys in my 50 gallon, which my dominate male will sometimes chase the other male especially when he may be getting too close to one of the females. I would continue to watch for any signs of aggression or stress. You should be okay as long as the tank is large enough and there are enough things in the tank to break line of sight.

The chasing has gotten alot worse recently, also, 2 of my platys tails have been split today and my betta fin has been completely seperated. I've rang the pet store and they said they won't take him back due to the chance of disease, nor do I have any friends or wish to give them the fish. The only way I can keep my other fish healthy is to euthanize the Molly but I feel cruel. Anyone help???