4 Love of Baby Chickens, looks like Chickerdoodle13 gave you some sound advice.
Betta are wonderfully hardy fish and can be used to get an aquarium established for other fish. Just remember that the process of getting the bacteria established is a slow one. Adding just a couple fish that aren't too big at a time is a wise thing to do for a small tank like a 10 gallon. Its pretty easy for the water chemistry to become dangerously toxic fast when the water volume is small. Its frustrating taking it that slow, but its more frustrating to get excited about new fish and have them die.
You could do both at the same time if you wanted to, but worrying about just one new tank at a time has its benefits.
The water chemistry will be substantially more unstable in the 2½ gallon, just because of the smaller water volume. Here's another idea that piggy-backs on Chickerdoodle13's. You could use the Betta to get the 10 gallon established. When you're ready to get what ever fish you'd like to try in the 10, then use some of the gravel from the 10 and enough water to fill at least ½ of the 2½ gallon from the 10 gallon. By doing that it will make establishing the 2½ gallon a breeze. Its a technique called seeding. The gravel will have the beneficial bacteria living in it and so will the water. The Betta will be much happier with the move to the 2½ that way.
As far as the mollys go, yes some of them do get to be pretty big, so do your research when you're looking at the different varieties. For a 10 gallon you should avoid the sailfins. Besides the fact that they can get big, their lovely dorsal fins have a tendency to flop over when they are living in cramped quarters. I've never heard of an instance where the flopped dorsal ever recovered its erectness, but that doesn't mean it hasn't happened. You could probably get away with a group of 3 mollys in a 10 gallon, and 5 would be pushing it. Its difficult to have too much of a variety of colors in a small tank.
Like Chickerdoodle13 said, platys are similar looking to the mollys. There's more color options in platys than what there are in mollys, too. Just something to think about.
Silkiechicken, sounds like you have a nice variety of tanks, too. Yes, the BN haven't totally caught on in the fish stores yet. Once they figure it out though, they'll be happy they did. I don't have any more available right now. I guess I'd recommend checking with members in a local aquarium club, if you're not wanting to pay shipping. If shipping isn't a big deal, there are places online that you can usually get them. If you want tips of some places to look, give me a PM and I'll tell you of some that I know of.
Tamsflock, you are certainly welcome, and thank-you for posting the kind feedback! Aquariums have been such a wonderful part of my life. I enjoy sharing the joy of the hobby with others. Not to mention, I've met some wonderful people along the way!
About getting this topic split up. I apologize I haven't gotten it done yet. Life has been keeping me quite busy lately. Maybe next week.
I hope all of you have a wonderful AND SAFE Labor Day!