You could get away with a 5 gal "fuge" for chaeto, you are pushing the practical sizes for your sections in a 20 gal sump. The practical volume total for a 20 gal size sump is 10 gallons, you want it to run ~ half full. Don't know where that 40% thing came from, but it is the first I have heard of that amount. 10% been thrown around for some time. So you are not really gunna have 10 - 15 gallon "fuge," in any practical way in a 20 gallon tank. And do not squeeze your pump for the sake of a larger "fuge", unless you have an ATO, and even then it will be running constantly, as tanks average ~ 2 gal a day evaporation.
You don't need reactors on a new tank, as you don't know how the tank is going to run. Probably don't even need a fuge at first. Growing pods is only really a benefit if they are being raised in the DT, as not many-- if any, are going to make it up to the DT from the sump.
You are building a sump, fine. But a 20 gal is really small for a 3 section sump, a 10 gal is even worse.
Jim