How to make my dogs get along?

I have a 3year old 'Shitzu and Maltese mix named Angel' And a 'lab mix with rat terrier named Ariel' that is 3weeks old.. well Ariel try's to play with Angel and she keeps snipping at her and barking at her..I just recently got Ariel from the A.S.P.C.A. & she was great with other dogs..So is Angel.. How do I stop making Angel stop barking and snapping at her?