I currently am sitting around 164/175 inventory with 5 or so Large Adamantite/Scarletite and 4 or so Major Admanatite/Scarletite each. I have not maxed it out yet, but you certainly need to invest in some space as you gain more RS gear.
As for 2* gear, it definitely depends. If it's something you could easily combine (ie drops a lot), from a Realm you're lacking sync for, and is a weapon or weapon-type you actually use I'd keep them unless you're really tight on space. Personally, I don't keep most 2* as I've combined any of the ones I've really wanted.
For example, I have three 4* Mythril Helm (IV), I'm not going to keep any more low rank IV melee gear because I simply won't use it.
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