It depends on the ones you are speaking about. Any that get HOT you would not want inside where they would touch a flammable material.

Of course not! depends on size of bulbs but mini lights n/p and or LED's

Sure! I do it all the time and I sell my lights to people how do the same thing. Here are some links to the lights I we use inside.

BLISSLIGHTS OUTDOOR INDOOR

INDOOR OUTDOOR KALEIDOSCOPE LED LIGHTSHOW

You can use outdoor lights inside but not indoor lights outside.

Yep, what they said! Outdoor lights can come indoors with no problem (assuming you check how hot they get and place them accordingly), but indoor lights are not sealed for the moisture they'll encounter outdoors, so they aren't safe out there.