lol a free mantis is a good mantis. especially if it ends up being a G. smithii.
for your current situation i would highly recommend leaving it in the 20gal. it can have a few co habitants after a while (not for now... let it get comfortable) but nothing that might eat it or attempt to. like lions. or eels. or sharks. lol see why the 180 is a bad place for it?
also, it very well may get lost in the 180 and you might not see it often if at all.
am i sure about the glass? 100%. like i said only large peacocks (6 inches and up) and G. chiragra (adults over 4 inches) are capable of smashing glass. im pretty sure yours is not a chiragra anyways.
can you feed it shore crabs? sure. collect a few crabs (ten or so) and throw them in a bucket with a bubbler. I think that would keep them alive for a while. Don't feed them to the mantis right away. let them get rid of whatever toxins they might have in them. try to collect from a clean spot. don't add too many in the tank at once as they are not used to hot water. Id only add one at a time. Or you can try to acclimate them to hot water... Im not actually sure how that would go.