Bettas are raised in jars and not with other fish. This means that when most fish are learning manners and behavior around others in the tank they are shown that they are supposed to be the only ones in their water. So it is a shock to them to be put in with other fish. Some of them do adjust very well and will accept other fish within the limitations of the fish that they are compatible with. Others depending solely on the personality of the fish will kill every other tank inhabitant to maintain what they believe to be the "natural order of their existance" and cannot be blamed for the way they were led to believe that life was supposed to be.
The answer about depending on the individual fish is therefore really the right one but I would NOT recommend at any time having them with fish that they do not get along with or that are related to them. There are also a number of fish that while I have heard of a few instances of working there have been MANY more cases of them not working as they aggravate or harm the betta as he will not see them as threats and they are nippers or in some other way will put the betta at risk.
With a tank your size you could definitely put more fish in but care MUST be taken to keep the tank from becoming close or anywhere near crowded. Any time you have any semi-aggressive fish (and the betta is one of them) you cannot crowd them or they WILL become aggressive just to protect their territory and if the betta is anything it is territorial.
What you need to do is get the peaceful fish settled in the tank first and let them establish their territories and then add your semi-aggessive (betta) and let him establish with what room is left. This is the only way to have half a chance. If the betta is the first in the tank he is going to think the WHOLE tank is his territory and you will have trouble.
Fish I would avoid at all costs in a tank with a betta: Gouramis, Sharks, Barbs, Mollies, Swordtails, Platies, Guppies, Any fish with long and/or flowing fins of any type, All tetras with the exception of the Cardinal Tetra, Danios, Shrimp of any kind.
Fish that are generally good tank mates: Catfish, Plecos, Otocinclus, Loaches, Rainbows, and possibly a snail.
Hopefully this has been of help to you and please do not hesitate to ask if you need any further advice.
