We have a 12yr old cat, 10 year old border collie, to that we added one yorkie, and like you later felt he needed company to play with. He's ok with the cat, they keep clear of each other mostly. As for the border collie, he's so clumsy, I'd never leave him alone with a small dog. He has a lovely nature, but unlike most border collies, he's a klutz.
Once Caleb (yorkie) was 7 mths, we added another yorkie. It certainly did change the dynamics. It is more work, more expense. Caleb is very bonded to me, so I've had to work hard to give them both the time they deserve. To be fair to both, he's so laid back it would be easy to over look him a bit, and she lets you know she feels jealous vocally. Yet his feelings are easily hurt, if he feels I've given her too much attention. Caleb and my routine, had to change, he used to love slowly waking up in the morning, having a massage, then belly rub. If we try to do that now, Pip throws herself next to him and insists on joining in (so I try and do both, at the same time) She wiggles and jumps on Caleb, she has so much energy.Caleb is like Eeyore and Pip is like Tigger.
They are both adorable, but very different in their needs, ie she needs more physical stimulation than him. He needs more mental stimulation than her...yet them both need both.
I'd still do it again. Pip is such a cuddlier and adds to our family. The extra effort is worth it. Pip and Caleb play so well together, which is what we wanted.
There are definitely pros and cons.+
__________________ Caleb   Pippi |