About The Rarities[]
I believe that the rarities the units can be obtained at should be listed in the "Source" section, perhaps in parenthesis (especially the ones from the start of the game, since the 2.5 update made it so that those units come at four-star instead of two-star).
About The Term "Free Units"[]
I feel like calling them "reward units" would be more accurate, since all of them are rewards for something, be it something that happened in-game or outside of the game that's related to the game. And this way, people won't confuse them with free summons.