I personally wouldn't add anything else to a tank that size but you may be able to add some ghost shrimp. Shrimp may become betta snacks though. In 5 gallon tank you really shouldn't add any other fish. Bettas are territorial and aggressive in nature. 5 gallons doesn't provide a lot of room for setting up different territories or safe places in case a fish decides to get aggressive. Fancy tail guppies, the fish you suggested putting in with the betta, aren't generally great tank mates since their long tails may cause the betta to think its another male betta. This will cause aggression towards the guppy. Bettas don't need tank mates. They are not schooling fish and won't get lonely. I would let your betta enjoy his nice big tank by himself.