I have recently got a second dog, a small terrier. My first dog is a terrier too. Oh, the madness of it 
They spent the first few days avoiding, then they have been playfighting. That sort of wrestling, open jawed fighting, no real contact or snarling. The only thing is it seems to have escalated a little on the new dogs side and he is using my poor old dog as a raggy doll! It still appears to be play and no harsh words have been exchanged - yet.
My view is that i should stop them and i do, but i don't want to be constantly telling new dog off, he is a rescue (six months) although to be fair, he's pretty confident! He is a terrier, what can i say. My DP thinks i should just leave them to it and then if it does escalate to a full on fight one will put the other in its place and a peace will prevail.
They are both on my lap as i type and its peaceful, for now!