Tropical Fish tank set up?

A 10 gallon is too small for a community tank. You'll really need that 20 gallon for a community.

Mollies need at least a 30 gallon, so they won't work.

There are smaller relatives though with a similar body shape - Platies - that would work in a 20 gallon. Platies are fairly peaceful. They'll need at least 3 of their own kind. They are extremely prolific. Females store sperm and can give birth to several batches of 20 or more fry once roughly once a month. If you don't have room for all that fry, you might consider getting all males. If you do mix sexes, get at least 2 females per male, as males are very hard on females and will harass them constantly to mate. They do this to the point that if the sex ratio is even or skewed to males, it can effect the health of the females.

Platies require alkaline water, by the way - they won't do well in acidic water.

Corydoras Catfish could work for the bottom. These are shoaling fish, and need at least 4 of their own kind. They will also need sinking foods, and do best in a smooth sand substrate.

You might think about a Gourami for your center piece fish. You could get just one of these. These are closely related to Bettas, and have attractive colors and individual personalities, and are fairly smart as far as fish go. A Honey Gourami (Trichogaster chuna) make good community fish. Alternately, a Dwarf Gourami (Trichogaster lalia), while not quite as peaceful, can still make pretty good community residents. Choose 1 species of Gourami, don't mix with any other sort of labyrinth fish (so no Bettas, Paradise Fish, or other Gourami in the same tank). And only do 1 male per tank, as males are aggressive toward each other.

With 10 gallons you don't have a lot of choices as you can only have a max of 8 fish and most tropical fish do better in groups of 3-4 or 5-6, etc. Female betta need groups of 5-6 and 20-30 gallons for them.

So if you want a little variety I suggest 20 gallons and get 4-5 guppies, 6-8 neons or zebra danios, 3-4 mollies. Just keep it at a max of 17-18 fish total.

You could keep some balloon belly mollies in a 20 gallon, but they should really be all the sex. You don't really have room for them to breed. THE fry would overwhelm the tank on the first batch. the females would have no room to escape the males.

You could try endlers or guppies or platties in a 20 gallon. You could try to keep a single female betta with them, but its best to have a back up plan in case it doesn't work.

Ten gallons is too small for breeding, and is limited on the species it can acccomidate as well. it can hold

5-6 same sex guppies, or endlers live bearers

Or

6 cherry barbs

Or

5-6 Kuhli loaches

or

5-6 harlequin rasbora,

If you want to breed mollies you'll need a 30 gallon tank.

A sorority of bettas is best done in a 20+ gallon tank.

I'd look into guppies (male only unless you do a 20 gallon), endlers (again, male only unless you do 20 gallons), smaller tetras, barbs, danios, or rasboras (2 inches or less, no more than 7 per 10 gallons but also no less than 5), cory cats (20 gallon min, 5 per group).

10g is very limited for options, even a 20g. If you can, try get a 20g. Mollies need at least around 20g and if you plan on breeding, you will need another tank otherwise the babies will be eaten. The 10g might be good for this. You also need to know where these fry will go once they're older. You will get overstocked. Alternatively you could leave them in the main tank and let the strongest ones survive on their own.

Other than mollies, 6 cories (preferably dwarf/smaller cories) would be ideal for the bottom part of the tank. They are peaceful and fun to watch. You will need a sand substrate though, otherwise their barbels will get damaged. They spend all their time sifting and digging through the substrate.

Lastly, depending on how many mollies you get, you may have room for about 6 of a single type of schooling fish. You could try neon tetras, ember tetras, black neon tetras, or perhaps ember glowlight tetras. I have glowlights and black neons. They are easy to care for, quite hardy. Alternatively instead of schooling fish you could add a single female betta. This should work, however be sure to have a back up plan in case she doesn't fair well with other fish. Also again depending on the other fish you get, you might also be able to have 1 schooling type fish as well as the betta, mollies and cories. But the schooling fish may possibly nip the bettas fins, causing a stressful situation for her. Also this could be overstocked, depending how many fish you get, so be wary.

I would recommend guppies. Get a 20 gallon and get 1 male to every 3 female guppies. When I set up a new guppy tank I always ask the pet shop for guppies that are already pregnant. They will breed very fast so set up a 10 gallon for the baby fry. I have found that neon tetras are a great addition to any aquarium, and also add 2 or 3 cory catfish. You can sell or give some of the guppies to pet shops and friends if you get over-stocked.