lol.. dont think you can get them in captivity which are black, but no idea really.. they need a 4ft x 2ft tank to live in.. which is the same size as for the snake.
its chameleons which change colour, and are pretty cool to watch too!
Joga.. nah they arent too bad, they dont really get big enough to cause problems.. only 6-8 foot, and all they do is give you a hug!
Corn snake, ball python are probably the easiest and cheapest, which dont grow too big, they could probably live in the tank youve got.. type in (snake name) caresheet into google, there are some pretty good ones out there.. you could try looking on www.faunaclassfields.com for snakes, and www.beardeddragoncaresheet.org to have a look at the type of care they need.
a 20g long is probably too small for a beardie, vivs are very easy and very cheap to build..
This took me a day or so to build, and will happily hold the boa for its whole life..