If you don't use the 20g for a sump, you can use it for a quarantine tank. If you are planning on adding more fish, you should quarantine them for about 3 weeks anyway, it would be a good place for them. You can just run a simple sponge filter on the tank. In fact, you can keep the 20g under the 55 and then it would be easier to turn into a sump later too if you ever decide to go that way. If you get a sponge filter and pump you can start cycling it in your 40 now.
My kid's 20g long with sand:


My kid's 20g long with sand:

