I would start right up on the ace, no break. Although it is a very short ester, it will take a little time to build and during this phase, the long acting Primo will start to fade. And yes, you can mix any oil based gear in the same syringe. In fact, what I personally do when using a long and short ester, is break my long ester shots down into EOD or ED so that the shot volume is the same each time. This way, I don't have to remember what I did before, and I don't have to worry about larger oil volumes sitting in a muscle.