Having a second one is wonderful. We got ours closer together (about 3-4) months apart, so the getting a long wasn't an issue. That said, you should be able to maybe let him visit with the new puppy prior to making your final decision, depending on how the owner feels about that (i.e. young pups, don't have immune system built up yet, etc.).
I'd also just look to how Mossimo has been with other dogs to this point? Does he love playing with other dogs? Is he really high energy? So them maybe a lower energy dog would be a good fit, etc.
In our case, our first one was pretty shy, so I wanted our second to be bolder. It has worked out great. Whenever she isn't sure of a situation, he leads the way and she follows. They play together all the time and I think they truly enjoying having the other around.
You mentioned about having one crated and the other free. That has not been an issue for us. My older one has been litterbox trained pretty much from day 1. Our boy never was interested in the box, so he is crate trained. He isn't a year yet and has been quite the potty training challenge, but he is so much better now. But - I do keep him crated if I'm not home or if he didn't "go" when I put him out and then try again a little later. It's never been a problem, because that's what we did with him from the beginning. No ex-pen - but I have baby gates setup so he can have 1 room or 2 rooms to run in, etc.
The other dog just takes a nap when he's crated or sits on top of his crate with a toy until I let him out..lol
Best of luck in your decision. I have to say, we love having two and can't imagine what our lives would be like if they weren't both here. I just keep trying to stifle the urge for another some day...