the pl*co, return... they are cool fish, but get very big...
to help the betta, buy a bunch of plants, some for the bottom, and some floating, id get some anubias for the bottom, and some indian fern for the top, this way you wont need much light to keep the plants going... bettas like lots of plants to hide in and make bubble nests, and it cuts on water movement... also, if there is no filter (you can buy a microfilter for one that small) then make shure you change about 1/2 of the water per week, if the betta seems sick, or its fins look ragged, then change the water again...
good luck!