male betta cant mix with tetra.
betta is aggresive and it will attack small size tetra.

i think ur undergravel filter is ok.
but dun use the hang on filter cos the powerful power head will suck in the tetra especially in the nite and when u off the light.

my all tetra were killed by the power head.
so i hate to use that kind of filter,
i advise u use UGF or submersible filter.