you can get cheap used exo terra 18 cubes for 40$ on internet... good for 2-3 green tree frogs
50$ for 18x18x24 which are the #1 most perfect size tank for pretty much any frog in the hobby! good for 2-3 gray tree frogs
but yeah, as a beginner, you really do not want to mix frogs... there are absolutely no pros for the frogs, only cons.
after a couple years of successfully taking care of your animal then maybe you can start exploring.