Mudskippers are BRACKISH, so there are no land lizards or amphibians that can go with them as it will dehydrate them and kill them. If you are so bent on getting a Mudskipper, why can you not just give it a species tank?????? A 20g will barely work for 1 Skippy, NOT 2!!!! A 30-40 gallon is minimum for life so you will be already looking at upgrading just for one Skip. If I were you I would take people's advice here and not try and push it otherwise you will end up with dead animals and people will get tired of the Oh No posts.
Also, I find it highly rude that your dad was kind enough to buy you a tank yet you call him cheap! Have some respect for your parents and THANK HIM for being kind enough to buy you a tank!!!