You can double-check with Dr. Roy, but an Odontodactylus havanensis would do just fine -- very little risk of breaking glass. To be extra-careful you can put a layer of acrylic on the bottom of the tank.
A spearer like P. ciliata would do great in a 20.
As far as "special requirements", most common mantis species like "ideal soft coral with inverts" conditions -- excellent water parameters, moderate flow, moderate (PC fluorescent) lighting. They tend to be messy eaters, so a refugium is a good idea, but not necessary. I put skimmers on my mantis tanks; YMMV. The Odontodactylids (and other large species) like PVC caves to lurk in. That's about it!
Dan