If I had it to do over again, I would have gone with the 22 inch WMS. The 18 is a bit small even for a large rack of spare ribs. I bought a package of wooden skewers with very sharp ends to you could use those to curl the ribs up a bit and stick a skewer through them to hold them in place. I see no reason not to buy some rib racks if you wish. I have intended to and just have not done it yet. Might get 3 on each grill for a total of six that way, not sure. The spare ribs I buy are so big I can barely get one rack on the top (or bottom) grate and get the lid closed. They are large and meaty, and always taste real good.
Good luck.