If it's under a month I can't really see a problem. Just watch your water and do frequent water changes and lots of air. I currently have a 7 inch Rhom in a 33 and it is doing amazing. I just bought a new 230 gallon and am going to throw my Aro and 13-14 inch ID shark in. That will free up my 120 for the Rhom to reside in for the rest of his life alone.
Personally, I've found all my serras to do best in tanks that were not overly big for their size. When I put 6" rhoms, spilos, or whatever into a larger type tank, they tended to get "lost" in the tank and become very skittish or shy. But in tanks that were smaller but sufficient, they were much more interactive and agressive. I would personally leave him in the 20 gal and when you are ready move him into a 40 gal tank. And in a couple years move him into a 60 gal or so.