First of all, you wouldn't have to shorten that Schmidt 2010 refill to use it in a rollerball kit. You may have to shorten the spring a bit, but there should be enough slop in the spring to just use it as is with no modifications.
Second, the Jr. Retro/Gent/Statesman are the same length as the Baron/Sedona, they are just a bit fatter, like maybe 1/32". The nib holder on the Juniors is round, hexagonal on the Baron and triangular on the Sedona. The diameters are very similar. I doubt you would notice any real difference in feel. However, if the Sierra cramped your hand, I'm guessing that any of those could easily do the same. The nib holder is a smaller diameter than the nib end of the Sierra. Was the El Grande you tried a ballpoint or rollerball? If it was the ballpoint, measure the diameter where it feels too big. It may be larger than the EG rollerball nib holder. The EG rollerball nib hold and the Churchill rollerball nib holder are the same size and I actually find them more comfortable than the smaller pens. I do not have large hands, just average size.