I would NOT put loaches in a betta tank.. they thrive in completely different water conditions.
I have a tank just over 30g.. gravel, loads of wood and low light plants.. I wish I had the money to begin a worldwide trade in Australian native Darwin Shrimp, as they are by far the -best- shrimp for a betta tank I have come across. I have them, and I love them... Anyway..
Cherry shrimp will thrive and breed, if you have a LOT of moss in there, and they will become your betta's favourite thing to do and eat, ever. Watching a betta hunting crafty little shrimp is amazing, the bettas are so happy and the shrimp.. well, the fast ones can live a very long time. I had some live more than a year before the betta finally ate them. And that was a massive, incredibly aggressive plakat. A couple even outlived the fish himself.
I have two bristlenoses, the Darwin shrimp and (for now) one fancy betta in there, and the betta is having a total blast. He occupies one heavily planted patched of java fern on one side of the tank, the bn's hang out on the other planty side. The bn's will grow to 15cm each, so I don't think it's understocked at all.
Corys are wonderful, and if it wasn't for my love of bn's I would have had a whole school of them.
Just personally speaking? I would not put bettas in with other fish like guppies, mollies, etc.. Mainly, as they have different ideal water conditions, so whatever I aim to please, I am not going to make everyone in the tank perfectly happy..
Basically, I'd go for whatever species are happy to live with soft water, lots of wood and a square ton of SE asian/tropical low light plants, because my betta's happiness comes first (yes, I am a bit 'specist' lol). If you're in America, you probably have a few more choices than I do, as our QT laws mean many species are hard to get or impossible here.
That's just my opinion, however. I am sure there's lots of others. :-D