Future tank size and possible tank mates?

Hi there, so I may be getting two 2-2.5" IT on this upcoming Tuesday or Wednesday. These will be my first dats ever and my first real big time fish in terms of size potential. They'll be going into a 55 gal for the short term then probably into my 75 gal once 4-5" then probably a 120 stout down the road. My question is what is a good tank after that for at least two dats and should i go bigger if say I wanted to add something else such as a arowana. Would a 240 work? Does anyone just keep them species only, every video on them shows them with other large fish? I currently have five 1" spotted silver dollars in the 55 gal but I will most likely move them out so they don't out compete the dats for food. Would they make good dithers down the road? Also has anyone used a K1 filter with dats? I ask cause I don't know if they'll be freaked out with the constant motion of turning k1 media. Sorry for all the questions.

So I did end up getting the two IT dats today. They look amazing and already began chowing down on black worms. Once I get a new phone I'll post pics. There both about 2.5" in length. To answer your questions why 2, well I honestly couldn't really give you a great answer. I figured there cheap i paid $25 a piece, banking on the fact that the shop that sent them to my LFS was keeping them in the same tank so hopefully they will be used to each other (so far no aggression but its only been a few hours). Also **** happens so if one kicks it for whatever reason I would still have another. However before you jump on me further, if they don't get along my buddy who is into monster fish (has a 240 gal running) is down to take one off my hands. Oh to also add a bit on the tank question, I was a LFS and really like the 210 dimensions.