I currently have one female sailfin molly, plus a few fry, in a 7 gallon tank. At some point I might like to get another fish or a few, but I don't know what type of fish is best.

The fish's temperament isn't the best; I used to have another fish but she bullied it a lot. However, that was in a much smaller tank and she probably just didn't have enough room. She'd probably be nicer now that she's in a decent sized tank.

Other molly's, Swordtail's, small Gourami's, Rainbow fish. small Cory catfish. Just to name some. But, not all the ones I listed are safe with the baby fish. You would need to remove the fry 'baby fish" to another tank. Just remember when buying new fish to ask the sales person witch ones that can and can not go in the same tank. Any good pet store will be happy to let you know witch fish will be happy in the same tank.