This hasn't been mentioned yet, but with a smaller tank,stay ontop of the top offs. You will lose more percentage of water due to evaporation in smaller tanks. Even if it's a fowlr
tank, get in the habit of topping off every day. Another thing about angels, they are territorial and smaller tank means smaller territory and more aggression. To be honest,if have the 55g already, just start it up and use the smaller tank for a refugium or sump. If you know you want a bigger tank,just wait a while till the money is available and start the bigger tank.