We had our betta alone for a while until we had an algae bloom. Then we put in a Chinese algae eater and a catfish which are both smaller than the betta. He flare at them and chased them around a few times, but then everyone settled down. He tries to eat their food when we give them a pellet and will still chase them occasionally. The Chinese algae eater started chasing him back about a month ago. Nobody seems too stressed out about it, though, and no one has gotten hurt.
For years I kept several different bettas with Blue Moon platties, and they got along very well. A lot of aquarium stores can tell you whether or not a betta will get along well with different kinds of community fish.
Do make sure they have plenty of room to move around freely though, as that will cut down on aggression. I wouldn’t have less than a 20 gal. aquarium.
Male and female bettas are two very different stories. I had several females in a 55-gallon community tank, and they never troubled each other or the other fish. I also had a single male in a different larger tank. He didn’t bother most of the fish, but he tore up a few fancy guppies, and I had to exile him.
Oh yeah, male bettas and guppies do not get along well. Probably partly because the bettas find their small size easy bullying targets, and partly b/c guppies are notorious pickers - they pick at this, pick at that, pick at everything. They seem to find betta fins very attractive to pick at, and it no doubt pisses the bettas off.
My betta lived in a tank peacefully with several other fish for two years. Until I put a female betta in. Then he butchered the entire tank.
So, the lesson is, kids, sex is bad. 
I don’t know about mixing Bettas with other fish, but mine is doing just fine in a fairly small bowl. They do just fine in fairly small areas.