If you use a direct thread mounted silencer, in CT you can screw it on any gun you have that's compatible (thread size, caliber, barrel length restriction from the silencer builder). Be careful though, a can rated for 22LR will screw onto a 5.56 and 9mm rifle...5.56 will fit through the can, but the can will probably fail very fast. 9mm won't fit through a 22LR but most 9mm/22/5.56 barrels have the same size muzzle threads. That's why I say just make sure it's compatible and follow any other restrictions, like the builder may say don't put it on a 5.56 barrel that's shorter than 12" or something but that's not a legal issue, that's just because it'll damage the can due to the abuse seen by the muzzle blast/unburned powder coming out of the shorter barrel.
In CT you can't put a threaded barrel in a pistol made after the 1994 cutoff unless you have a CT assault weapon certificate for it...makes it difficult to put a silencer on them. That could be what the guy meant and didn't explain well.
He also could have meant that if you want to use one of the QD mounts, like a SilencerCo ASR, Q Plan B, or a DeadAir Keymo, you need to be careful about which muzzle device you choose for the intended firearm...like if you pick a ASR flash hider, it may put you over the evil feature count. But not all muzzle devices count as evil features, and most mounting systems have multiple versions (flash hiders, comps, brakes...) that all work with the silencer, some of which aren't considered evil by CT.
It is illegal to use a silencer on any gun in CT when hunting.