You can use all of the aforementioned or you can name the mob two to three times in order for them to stack apart, or even have an MVP or up use /spawnmob far away from the other mob and then drag it over there so it can help you in breeding. May I also add,
- One other word about stacking and breeding: even if you have four or more adult animals, don't try to breed more than one pair. If you already have a baby and breed another nearby, they'll stack.
It doesn't matter if you stack the babies- The amount of times you've bred animals is what matters, not the amount of animals in the pen. So if you breed the mobs and the babies stack, you can just keep breeding them when they're ready to do that again. The more babies there are (whether stacked or not,) the more they'll grow up and will be able to breed themselves with other mobs. This works on just about any mob, not just sheep.