Upgrading to 75g, Can I use my 55g as a sump?!

Hey everyone, I'm in the final stages of upgrading to the 75g, everything is looking great and healthy. I knew that I wanted to put a sump on 75g and just always thought " do your research" and eventually get one ( actually build it ). So I was going to use my 20g acrylic tank, but now I'm thinking that I should use my 55g. I plan for a couple of months I would have to use my canister filter and have heater inside the tank which I don't mind, then add the sump. I read that the more water volume the better and more stable of a system you will have, and a 130g system sounds promising. But is a 55g just to big for a 75g reef sump? I know I want a majority of it to be a refugium because I am interesting in growing mangroves and macro algae. Also I'm not going to have a deep sand bed in the 75 because I'm not crazy about the look of it anymore and getting nasty on the glass. Instead I would have a deep sand bed in refuge and have copepods and all. I would also plan on having the lights on reverse to the main display to maintain Ph. I know I can do all of this in a smaller sump, but the bigger the better right?:):confused: