A 20 gallon isn't even really big enough for a single adult tokay. Also, tokay are really not a good beginner species. They are quite shy and can bite extremely hard. They cannot be kept with any other species as they are very territorial. Mixing species in one tank is not a good idea no matter which species, especially when you're talking about something as small as a 20 gallon.
I think that you might want to read through a lot of the older threads in these forums to give yourself a really good idea of what's required with any particular species before you try keeping one.