If you are doing RTS game with AI coded in blueprints (instead of blackboard), make stress test see how many units you can handle at once. Also for RPG, do such test.
Blueprint only AI works fine up to about 50-100 units, then it lags. Esp collision and animation. are heavy on CPU with bigger number of units
This is reason blackboards were implemented, to make more units that 50 or so.
Use blackboards if that is case for your game.
And formation is only matter of getting leader world location, getting its forward and right vector, then some simple vector math.
For formations (with vector math):
Store all formations in arrays of offsets, each unit gets offset that is relative to leader orientation (forward and right vectors multiplied by some distance from leader.
Then you get leader position and add that vector to target of formation member.
To change formation read different row from array.