O. scyllarus hits a max of 18cm. Large specimens will be able to break through 1/4 inch glass easily. There's a few ways around this problem: 1) get a big tank/ a tank with thicker glass (1/2 inch would be safe but may be overkill) 2) go acrylic 3) take a chance, use a standard tank but line the bottom with strips of acrylic and apply a bead of silicone around the edge of the aquarium up to about half an inch. I chose option three in a 75g tank. It should be noted that tank breaks are actually very rare and of those rare occurences, it is super rare to have a break anywhere but in the bottom pane. breaks usually occur when a mantis is digging and wants the stupid clear obstruction out of the way.
it's a big bug and usually an active species so imo 30g minimum. I have mine in a 75g (60g display) and I find it uses the whole tank. gives the tankmates plenty of room to hide as well.
tankmates is an iffy issue with nothing proven. To increase chances of your tankmates living, think carefully. It is a given that fast water column dwelling diurnal fish have the best chances. Damsels and wrasses come to mind. benthic fish like dragonets and gobies are likely toast; seahorses probably would be too. a well-fed mantis should have no reason to try to hunt down a fish. Bigger tanks = more hiding places which i think is a big deal. what worked for me was to introduce a damsel first... the mantis tried and failed to kill it and eventually gave up; from that point on Ive added a few wrasses, and a lyretail anthias. It seems that once a single hard to kill fish breaks the mantis' spirit for fish capture, that's it. I haven't tried benthic fish though. obviously no crustacean killers like big wrasses, puffers or triggers.
if you're thinking of corals, note well that you cannot have high lighting on your tank. Odontodactylids are susceptible to shell rot which is caused by overexposure to light; males are even more susceptible. You could easily keep the lowest light SPS, most LPS and softies though. That said, I went against the grain and have a 48" t5ho and a 250W halide; this would generally be considered too much, but I've figured out where the mantis stays 95% of the time so I just reconfigured my rockwork carefully such that he is in the shade from the majority of the light in that area. Once you know your animal, careful planning may allow you to accomplish out of the ordinary things for a mantis tank.
here's a link from the authority on mantis shrimp, Dr. Roy Caldwell. I'm not sure if you've found the mantis shrimp forum on RC, but he posts there and there is a lot of good info there.
http://www.ucmp.berkeley.edu/arthrop...me=o_scyllarus
you'll notice I threw in a lot of anecdotal evidence; usuallly not my style, but mantids are very peckish creatures. what you experience WILL vary individual by individual. you couldn't pick a more personable creature that doesn't have a spine. whooo, long post. let me know any other Qs
edit: i apologize if ive made it sound more complicated than it is. long story short, they are hard to kill animals, have very little in terms of demands, yet are very rewarding pets. Also, if you look up their actual biology a bit, you will be amazed. i guarantee it.