This isn't about the nfa items.
I have a few questions.

10 inch or 12.5 and why?

Rails with an internal opening large enough for common suppressors? Which do you like and why.

If this is 556, keep 10.3 to 11.5,with a can is the sweet spot IMHO. Gets too long otherwise.

The can will get very hot quickly, I don't recommend buying one under a handgaurd.

I would do a mod 0 or cqbr clone

I like the Aero Precision quantum m-lok rails.  Internal diameter is 1.7", which is big enough for anything except a salvo12 or an oil filter silencer.  They're cheap.  Including the barrel nuts, which have to be ordered as a separate item, I just picked up 2 of their blems for $140.

I can tell you that the PSA handguards are 1.375" inner diameter.  They're too small to tuck anything inside them other than a slimline flash can or a tiny rimfire silencer.

As far as length goes, there isn't a ton of performance boost from 10.5" to 13".  This chart was made with M855 63gr.

Just a couple bits of advice: I don't like rails, and I wouldn't extend the handguard past the muzzle.

Early in the 21st century we started getting away from rails. Maybe when you typed "rail" you just meant handguard, but the big, bulky, abrasive quad rail systems are almost gone from the market for a good reason. Modern modular systems that allow you to attach rails only where you need them are lighter, and significantly more comfortable to handle.

As for the length of handguard, it might look really nice to have the can tucked in under the handguards, but there are a couple of practical considerations that go beyond aesthetics: The first consideration is that more and more suppressors are going the way of a quick attach mount. Since most of them are more than just a pipe thread adapter, and function as a form of muzzle brake/compensator/flash hider, you will want to be able to get to for cleaning. I wouldn't want to try to run a brush through holes in the handguard to get to that adapter. Then, there's going to be that one time when, for any number of reasons, you end up shooting the rifle without the can on it. Muzzle blast inside the handguard is not a good idea. There are tons of pictures all over the internet if you need to get a good visual as to why. Just use your favorite image search engine, if you need to see it, but to summarize it ranges from handguards that look like peeled bananas to pics that require you to acknowledge their graphic content before viewing the gore of flesh that has suffered equally.