Betta in a 5 gallon Tank?

with a five gallon tank I'd avoid a large filter cause thats over doing it. There are some filters that use the motion of airbubbles ping through them to pull in debris and trap it for cleaning, these types of filters can be difficult to find though. A filter works in conjunction with 20% biweekly water changes. Your beta should be fine with or without a filter because they can breath air like we do and absorb oxygen from the water like other fish. As far as other fish and shrimp I would say try it out. I remember a friend thought his beta (siamese fighting fish) would beat up my cichlids, boy was he about to be wrong before we broke up the impending fight.