Remilia has a really slow leveling speed unfortunately. Also, members in the active party get more exp than those in reserve, and they get more exp than those in the SDM.
EDIT: :V poster above already responded to this part. This thread moves fast!
For some reason I get the feeling that my entire post wasn't read (probably because it is on the previous page). The rules I set for my challenge run were a normal run through of the normal game (first 20 floors) with only game necessary characters. Because it isn't a ng+, I won't have 16f or 18f until their respective floors, and I definitely won't have Meiling since she is an optional character.
No matter what I do, Remilia will have to first slot tank until at least I get Eirin, who is the same or worse as Remilia in hp, defense, mind, speed, and leveling requirements, but has superior affinities. If I use 16f as main tank, her leveling progression is much worse than Remilia's, not to mention her speed. The only thing that she is better than Remilia would be her mind and again affinities.
She also happens to be superior to 18f in everything except mind, (9 vs 10), and affinities but she also levels faster than him as well.
I feel like affinities aren't especially hard to raise around 14f and higher when you actually start needing them.
So after thinking about it myself, I probably will have to twink Remilia as a tank, considering I don't have any other choice. If 18f/16f can main tank, then I don't see why Remilia shouldn't be able to.