I've built several comped or ported shorties that ended up at Commander over all length. The trick is to use a CONED barrel, with a reverse recoil spring plug [ one that locks in from the rear ]. Otherwise, the bushing and the locking lugs limit spring travel, and you need to get complicated with the recoil springs [ double stacked/triple stacked/seecamp ]
If you just want to hang a 3/4" comp off the end of a Commander length barrel, to make it same as Govt Over all length, you can do pretty much anything you want. I still would recommend the coned barrel and the reverse recoil spring plug designs.
For several good ideas, take a peek at the Brownell's catalog on line. Look for something that says " carry comp". Or contact Gunnar @ Armco.