Ahh, darn pet store people....why can't they just give proper advice? :/ Gah, so frustrating....
To start off, I understand you were missled(I was as well in the beginning....in fact, I'm sure almost everyone was at some point)....but, if you really ant all of them to be happy and healthy in the long term, your going to have to re home almost all of those fish or
upgrade and buy a separate aquarium for your goldfish.
To start, your going to need at least
a 20 gallon for your mollies unless they're balloon mollies, which tend to stay smaller, and all male. If you have females that they were kept with males or have both male and female mollies, you'll soon be overrun with babies. Trust me, I've kept mollies before, it doesn't like very long
Also, all species, that I know of, apart from balloon mollies get to be at LEAST 4", and on top of ALL that they're big waste producers. So, you'll either have to re home them or start shopping for a 20+ gallon I'm afraid....
While the BN pleco is one of the smaller species, again, it will grow too large for a 10 gallon and, like all plecos, they produce a LOT of waste(or so I've been told, this is the one fish I don't know as much about). Personally I think you'd be better off just re homing him, though if your willing to upgrade, again, so a 20+ gallon then you could probably keep him....
The goldfish is the one you're probably definitely going to have to re home unless your attached enough to him to get him his own a 30+ gallon(Depending on the kind of goldfish he is, if hes once of those 'feeders' your better off finding someone who has a pond for him to live in....they can get well over a foot)and maybe a friend or two(I hear they're social, perhaps I'm wrong on that....). Goldfish should NEVER be kept with tropical fish, their needs are too drastically different for them to be compatible.
With all that said, onto the question. No, I wouldn't divide the 10 gallon for the betta and the mollies....thats a REALLY small space for ANY kind of mollie. With that said, I wouldn't put the betta with the mollies either....IME, mollies are pretty aggressive and I've heard one to many stories of them going after bettas fins.
Hopefully this has helped some, and I'm really sorry that you were missled....its so terrible what people will say just to make a few dollars off someone :/