Every time I add a new image or text layer, it goes the full length of the comp. Can I change it so that it only goes a few frames by default?
You can set it in AE preferences. But if you're dealing with a bunch of layers of text or stills, and they're supposed to have different lengths, it can get to be a real pain.
Many people find it easier just to use the keyboard shortcuts to set the layer's in & out points: opt/alt-[ for the in point, opt/alt-] for the out point. It works for multiple layers, too: just select the ones you want before you use the keyboard shortcuts.