I'd say Kuroko could maybe be considered a loli since she's small and pretty flat chested. But, not Mikoto...
As a workaround, you could use the child version of Mikoto. There
was a flashback episode (ep 5). If you recall, Kuroko, Uiharu and Mii Konori were in a bank... the little girls must have been like 7 at the time.
The bank was getting robbed, Kuroko fought against the robbers with her powers, and then...
Right when it looked like Kuroko was going to get killed by the last robber's equal speed marbles (like a dozen of them), Mikoto used the railgun from outside and totally obliterated the marbles in mid-air. But, nobody except the two of them knew it wasn't really her that did it.
Sooooo... you could use the child version of Mikoto if you want her, if you're willing to compromise. But, I really don't think the normal version of her is loli at all. She's noticeably taller than all of them, she's the oldest of the bunch and she has noticeable cleavage.
She could use the railgun back then though, so it's not a terrible compromise. And you could keep Kuroko at her normal age since I think she qualifies. Anywho, that's my two cents...