Glass cats are definitely not a good idea. Although they are incredibly nifty, they are also very sensitive. You'd need a 30 gallon tank, dimly lit. They are not good with bettas because they are extremely shy. They also need schools of six or more.
As Pataflafla said, 4 panda cories or 6-8 pygmy cories would be ideal. :) If you like a bit of colour combined with the wierd transparency of the glass cats, have a look at ember tetras. I have some in my sorority and I adore them. :) 6 of them should go well with a betta.