TT shrouds Size?

I have a Talon and Condor was wondering what is the smallest size TT shroud I can put on either rifle to tone it down. I want to shoot a 12 or 18 in barrel. Want to keep it stubby. I know the 24 in barrel needs a 10+ inch shroud. Thanks for your info. Kip P.S is it worth it to put in vented spacers? Your thoughts. Thanks