
I have an ex-pen which we used outside for our litters. However, with one puppy I'd be sticking to a crate, and one big enough to last beyond 'baby puppy' and a baby gate. Yes for sure, your existing dog should have a place to retreat to when she needs a nap, without being mugged, but actually that would really be up to you to control - if the pup gets above himself, crate!!
Another query about a play pen is with a puppy like a Golden Retriever, no play pen is going to stand up to him if he jumps at the sides I'd suggest.
Basically as you know, puppies need to be able to run around a lot during the daytime, so when you/your existing dog need a time out, your puppy would be far better being crated!! My opinion.