ya the guppies are not a good idea!! and with a 5.5 gallon tank that is way too many.
I have a 10 gallon with 1 betta, 6 neon tetras, 3 otos, and a snail and it works just fine. as a general rule, you dont want any fish that is colorful or has flowy tails that a betta could mistake for another betta. I even had 2 hatchet fish that i tried and my betta would just not stop flaring and wouldnt leave them alone.
for a tank that works, you want each type of fish to have a dedicated level of the tank: for the betta, the top; for the tetras, the middle; and for the otos, the bottom along with the snail. this way no one is in anyone else's territory.
for you, i would suggest 1 betta with 3 tetras, especially with only a 5.5 gallon. overstocking could kill all your fish :(