My boys are fine with no clothing inside or outside. They won't wear it, so I don't force the issue. I keep my house b/t 68 - 70 ... i figure if Im warm in a sweatshirt, they are warm in their own natural fur(hair) coats.
I don't let them stay outside for more than like 5 mins though, when its super cold, they pee and come in, its like 30 seconds, but on a sunny winter day they will stay out (on their own) for like 10 mins and be fine. This is my third winter with my boys and they are happy, healthy little guys!