Does Knights or anyone else make a rail system for midlength config barrels? I originally wanted the SIR #58 but the size of it is putting me off. I'd like to get rails top and bottom but would like to avoid bolting them onto the stock plastic handguards. Thanks.

KAC has a mid length handguard but it is longer then the standard mid length.  it will cover the gas block.  they also have a short mid which is shorter then the standard mid lenght. don't ask me why.

The PRI midlength is probably your only other option if you want use fo the original front sight block.

Armalite has a rail system on their website under new products. It is a RAS style handguard replacement, so I don't think it will free float.

That's fine, I'm not looking to start disassembling my newly put together RRA upper so non-freefloat is perfectly fine by me. I'll check it out!

The SIR side rails do not fasten to the polymer hand guard, they go thru it and run and fasten into a metal track from the upper rail assembly. When a grenade launcher is in place, the hand guard is naturaly first removed, and the side rails go back into the track and fasten via a spring loaded screw, and accept various launcher sights, the point beiung the polymer has notheing to do with the fastening ability. The #58 also has a dovetail built into the front bottom hand guard extention that takes a light or bipod, etc. without extending any lower that the hand guard it's self.
Good shootin, Jack