In my opinion, you are making your first mistake by looking for an animal to fit the tank. It's highly unwise of you to do that, and I can almost guarentee you'll lose interest in the animal if you do that.
Second, nothing that you listed will be comfortable in a 20 gallon tank. Even a corn snake will need at least a 30 gallon. Kingsnakes get pretty big, too. My adult ball python is in a 40 gallon tank, and I wish that I had something bigger to put him/her in because when you get everything in the tank, there really isn't a lot of room. I had him/her in a 20 long for a while and felt cruel.
How about you wait until you find something you really like, and THEN save the money to accomadate that animal, instead of just taking whatever will "fit".
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0.1 Irian Jaya carpet python
1.0 Hogg Island Boa
0.1 Ball python
1.3 Crested Geckos (harlequin/fire, dalmatian, fire)
4.6.0 Leopard Geckos (nrml, abno, htct, blz, lcs)
0.1 Western Hognose snake
1.0 Albino Lavender California Kingsnake
1.3 Bearded Dragons
0.2 Water turtles
0.0.1 Rose Haired Tarantula-Sammy
1.0 Umbrella Cockatoo (belongs to my mom and I)