You can combine anything you want together. However, the finished video has to match your project format, so something will probably have to give. If your project is widescreen and you insert standard clips, by default they will be shrunk to fit the height of the frame with empty black space on both sides. You can crop in to fill the frame and lose some of the top and bottom, or leave it the way it is, or crop partially, or many other choices. You'll have to make some choice to get it to fit as best as possible.

When you're done, the entire video will be the same format. You can't have it switch formats during the playback of the finished video. But you can use varying formats as source clips.