I am no expert, first and foremost, but here are observations/attempted help.
Firstly, the fact that it is red is not an indicator of what species it is.
There are hundreds of Stomatopod species, and many have widely varying color patterns. It could be a young member of X species or a full grown adult of Y species, for example. Where the little bugger was collected is going to be a far better indicator of what species it is, and if it is from the Australian waters, it could be some species that we Americans (the majority of people on this forum, I have noticed) have never seen.
My own little G. Viridis occurs from Vietnam and the surrounding area down to the north of Australia, so one paper online told me. G. Viridis has a more subdued color scheme, so it is most likely not what yours is,though.
That being said, I agree with MSL in that it will most likely not jump from your tank (though lids are always a good thing,right?). Diet is a strange thing with these guys. Some like snails, some crabs and shrimp, some seem to like frozen food , while others scoff at it.
My G. Viridis has a definate preference for snails, while my G. Platysoma pretty much ignores them in favor of the hermits that I put in there. he just went in to molt, so perhaps he is just being peckish...
My G. Ternatensis is ravenous, eating snail after snail, as well as a fiddler crab that I bought for her, so as to give her something to 'hunt'.
All of mine have taken frozen shrimp, as well as silversides. The G. Viridis used to eat cod, though now he refuses it entirely.
Ghost Shrimp were eagerly eaten by the Tern and the Viridis, but the Platy ignored them, except to eat one little one. As said, he was touchy before a molt, however.I have a half dozen snails and 4 hermits in there for when the monster emerges from his lair.
(I have also taken the opportunity to rearrange some bits of the tank while he is locked away, which brings me to...Getting whacked.
MSL mentioned that his O Scyllarus (Peacock) whacked him, obviously without much real damage (Good thing for that).
However, that species is capable of breaking glass, fingers and dowel-feeding sticks, as noted elsewhere. MSL's obviously wasn't in a bad mood, or perhaps there is some Jedi-Mind-trick involved to make it friendly ( "These are not the fingers that you are wanting to break..." )??? I definately did not hit MSL as hard as it could have.
Smaller Mantis are capabe of inflicting bad wounds, either breaking fingers, or as Dr. Roy has mentioned several times, causing infections from the cuts (smashers still have little vestigal spikes/blades). I believe that he mentioned a stab into the bone as being the cause of an amputation that one diver suffered.
A guy at a LFS got hit by a 3" hitch-hiker and suffered a nasty wound, with infection blowing his hand up something fierce.
Just be careful is all that I am saying.
The size will be dependant on an ID (cooming shortly after that site is back up, I would assume. It was still down when I looked).
As far as Bioload goes, I am unsure, though if you have a fair bit of Live Rock, and good skimming/filtration, that should not be a problem, as long as you remove uneaten food.
Once a mantis 'tosses' food out of it's burrow etc, it has said that it is done with it. Sure, there may be cases where one will eat again (I have fun pushing food back at my Viridis, just to see him toss it again, like we are playing ball. He stopped to munch a bit one time...), but you should always remove leftovers.
I have a truly staggering worm-bed in my first tank, so extra food doesn't seem to be a problem, along with several hermits that have grown considerably, since the Viridis doesn't seem to like the meat/work ratio involved with them in comparison to snails...
I am letting them (the worms) go into overload breeding, since I am farming them out to my other two tanks, as well as in anticilation of finally setting up my 55 gallon mantis hotel and reef tank.
Just my 2 cents (Well, more like 85 cents, but still...)
-Ron
PS:If you are really attached to that particular snail, move him to another tank. The mantis will et around to him in time...they are predators, after all. Good Luck.