I can't address your first question from experience because the poodles were here first. As for the second, the greys and the poodles do not usually play together, even when out in the yard at the same time. The Core Four (Perry, Minnie, Betsy, Edie) are out together, but play with me (retrieving). If Edie takes off and runs laps, Minnie now sits in front of me for treats--a behaviour we developed because I was worried about her nipping at Edie while chasing her (I didn't want to muzzle her because then she couldn't pick up her ball).
I won't add a puppy of any breed to this crew because I don't want to deal with the management of a houseful of older girls (10 and 11) in the presence of a little monkey. But it is just that: a management issue. If you're willing to supervise constantly, intervene when necessary, and crate the puppy the rest of the time, it can be done.
Several GTers have added puppies to an existing crew. I'm sure some of them will stop by and tell you about it.