Is it safe for my puppy to wear dog shirts?

I have a five month old Dorkie and I live in San Fran, Cali and it's getting super cold lately. My puppy was getting so cold to the point he refused to go outside to use the bathroom and would just do his "business" inside and in the morning and nights when it got even colder, he refused to move from his bed to do anything. Of course I have the heat on but I cant run it as much as I want because I'm a college student on a medium pay here, so I usually just roam around the house in hoodies without bothering to turn up the heat, but I felt bad because it was unfair to the little guy. So I bought him two doggie shirts, nothing thick like a sweater or hoodie, just a simple cotton tee for him to wear all day and now he'll actually go outside to play and he doesn't shiver or try to hide under the covers all the time, so I was pleased. But yesterday when I took him to Petsmart with me for some shopping, this one women went ballistic on me that he was too young to be wearing clothes and something about his puppy hair and I was scaring him this way and being irresponsible and all these things. So now I'm freaking out, is it bad to have him wear those little tees all day for the winter time? I don't want to hurt or scar him, but I don't know how else to keep him warm.