I totally agree with Becky, entry level server has to be the FOX. The MRT is a harder car than the FOX, so you'd be going backwards from the MRT to the FOX.
I acknowledge that the FOX does nothing to prepare you for the other single seaters, but the entry level server would be an introduction to race craft rather than vehicle control. Once drivers know how to behave on track you can let em loose with the more difficult vehicles.
So as unnatural as it looks, I'd suggest FOX > MRT > FO8 > BF1 for progression. Possibly combine the MRT and FO8, alternating on the second tier server. Then we have the pinnacle of sim racing, the BF1 tier (where drivers aren't just going round in circles)!