In a 15g aquarium, I kept a betta with a school of harlequin rasboras and a school of pygmy corydoras. Very nice planted tank. I've also kept bridget rasboras with bettas, but they are very small could end up a snack if the tank isn't large enough. I have also kept espei and hengel rasboras with bettas with success. I've heard different things from different people regarding livebearers. Guppies seem to be a resounding no (with exceptions), swordtails it's divided, platies are the same. There is always someone who will disagree with someone else. You have to do research and weigh peoples' opinions carefully.
I have a planted 8g with 5 dwarf sunburst platies that I may move one of my bettas to, but I have to determine which one has the best personality to try this with. Also, that tank is densly planted, so stocking can be more generous because the plants utilize fish waste effectively. You really have to consider the betta's personality. They really do have personalities, you know. Some of mine are really mellow and would be great in a community, others are not so mellow.
That being said, you could try the pygmy corydoras. Corydora pygmaeus only gets about an inch long. In addition, they are pretty drab, and unlike other most other corydoras, they school throughout the entire tank, giving a lot more movement in the middle and upper strata. I don't think even the most agressive betta would bother them much, they're very fast and can hold their own quite well. They are, however, rather difficult to find, and really do best with a slightly cooler water temperature. You would, however, find a good compromise for both betta and cory at around 75-76. Any warmer, and the corys don't die, they just become more sluggish. At least that has been my experience with them.
You
can keep a betta buy itself in a 7g, you know. There's no harm in it at all, and many here keep them alone in even larger tanks. It won't be lonely and if there are plenty of decorations (plants, rocks, caves, wood, etc), the tank won't look empty to you. If you're happy with one fish, then get a betta. If you really like the look of more than one fish, then don't try to force a community where none can exist because a tank may seem empty. IMO, bettas in community tanks are better left to those with more experience, or to those who at least have the knowledge to understand that a humane back up plan (tank for just the betta) should be on standby incase there are problems. Perhaps a betta may not be for you. Just something to consider. There are other options available for stocking a small aquarium.
Warmest regards,
llj