Electric RR and postwar 646 Baby Hudson conversion

Can't help you with the 646. But like you I have installed TMCC in a 2321 with railsounds, a 2343 SF with original horn. Also a 2360 GG1 with original horn, a 736 Berk with original whistle, and two MPC engines both Susquehanna- an RS3 no sound and a B8 with railsounds. These were done about 15 years ago and all still run fine. I also put TMCC in a 1665 0-4-0 and a B8 0-6-0 with Sound of Steam but sold them.  About 5 years ago i converted an 1872 General using ERR components including sound. All run so much better especially at low speed.

I've put them into a host of different locomotives, and even several AC motored steamers.  I don't see any special issues doing that one, I use the MTH tether components for my upgrades, that gives me 10 wires to the locomotive.  Since four of them are heavier and six are #26 or #28, I usually double up a pair for the AC motor's 3rd wire.  That leaves you four wires for light, smoke, and chuff.  You even end up with a spare.  I presume you're thinking of the AC Commander, that is an AC motor, right?