The short ARCA rail idea...
I tried it and found for short lengths it did not work. The torque applied while in a RRS rail clamp caused the entire metal of the rail to flex and deform like a tin can where the ARCA rail was connected. This needs to be a much more substantial connection like part of a solid chassis or rail or SEVERAL bolt connections to spread the load on an MLOK rail. One or two slots just WONT cut it. Then it gets in the way shooting offhand if you do longer rail to spread the load.
IMO a short ARCA feature needs to be built into a rail or you just get a crazy amount of flex from rotational torque. That or a 4+ MLOK section long unit to spread the load to multiple bolts and accept the big flat section will suck bad for off-hand shooting.
If you want to retain offhand shooting ability I would suggest a hog saddle clamp type device over a RRS ARCA clamp would be a better choice. It will be more rigid than a short bolt on rail and work for all your non ARCA rail weapons.
Just seemed to see your end goal and thought I might save you some headache I already went through.