if I dismiss a companion can i go back find them and get them to join me again?

Yes. Make a save before just in case they get stuck in the wilderness or something.

Why not just tell them to wait where they are

Because that's sometimes a pretty bad idea.

After so long they will return back home if you leave them waiting too long... i left two of my companions waiting for a few hours of game-play because i didn't want them to die and after so long i got a pop-up saying that had returned.

yeah i would tell them to wait in most cases just wondering