You could do a P grandis in that 20L for a while, but you should upgrade within the first year or so to a bigger/taller tank. The 18"x18"x24" exo-terra or zoomed reptile enclosures work really well for them. The tanks that are front opening are also easier on you to take care of your Phelsuma. Top opening tanks tend to lead to more escapes.
Substrate-You'll get a lot of answers on this topic. I myself use cypress mulch. Has worked well for me.
There are tons of species you could try. Take a look online and see what species are available and how much you want to spend on them!