I assume the term "fit" above means the helmet feels snug with no space between the pads and the cyclists head and the helmet can not slide on the head with the chin strap centered under the chin, fitting snugly so that opening the mouth wide the helmet pulls down on the head.
MIPS will help in a crash with a good fitting helmet and a lesser fit will interfere to some degree with the MIPS advantage as it would with a helmet with simple padding protection.
My three daughters, granddaughter and me with a 7 3/4 hat size and rectangular head can get a MIPS to fit.