Oh marine bettas my favourite fish! They are great fish, yes they will get max 8'' and they will be ok in a 55gal. They wont eat your fish but they can eat your pepermint shrimps and if you will add a small cleaner shrimp after adding a big marine betta, cleaner can be meal too but they wont harm anything else like big shrimps, corals, invert or small fish... In a 120 you can keep 2 of them so they will be out more often but if you are gonna keep tangs or angels or any active fish dont add marine bettas