Great advise from Gill - can't think of a thing to add.
My 2 can get really rough with each other - when we bought Lilly home she weighed just over 1lb and Zippper weighs about 18lb, I used to panic but they just sorted it out between themselves, if Zipper got a little to rough Lilly would cry and Zipper would back right off and look really worried.
They are now 20 months and 7 months and still play like crazy together and Lilly's only 1/2 Zippers size now.
