For filtration
AC50 = Aqua Clear 50 (I think thats the new designation for the big twin chamber one). For your filtration on a 20, I would use one of the big aqua clear's and instead of using the Aqua Clear media, I would jam one half with Live Rock rubble, and the other half make into a mini Refugium. Then add a Hand On Back skimmer like an AquaC remora, and a powerhead in the tank for extra water movement.
As for my cardinal, he looked lonely so I moved him over to the reef tank, where he still looks lonely. Now I have a Pair of Yellow Tail Chromis and a Saphire blue Damsel in with him. For about 6 months there were the same peppermint shrimp. Granted I have an N.wenn mantis and they aren't the most aggressive bugs, but a buddy on Grimreefers.com had a cardinal in with his Peacock, and for the longest time they even shaired the same burrow. FWIW as long as the mantis isn't a hyper aggressive species, and doesn't feel threatened, its only going to kill what it can eat. Keep it fed and it will mostly leave your fish alone. Thats not to say that it can't one day decide that the Emerald Crab Cake can wait, and go for a fish, and for that reason I wouldn't recommend keeping your favorite fish or an expencive fish with your mantis.