You can have any of your pet dogs working on the track at any time, however they will not be offered for combining. Each breed creates a particular amount of coins whenever they end up a lap, and the higher the breed the more coins they will get. As you level up, your optimum pet capability will increase, allowing you to put more pets on the track. You should attempt to have as lots of pet dogs on the track as possible, even if you are actively combining. More dogs gives you more coins, and this is specifically crucial to do if you need to step far from the game for a bit.