I have a stallion and he gets along great with our geldings. I guess it all depends on their individual personalities...same with mares. I have had a couple boss mares that would not give dominance to another.
If I were in your situation, I would sit back and really think on this. If I could afford the $3000 purchase, the cost of vet, farrier and feed for two additional horses, and have the time to spend with two driving horses without taking away from the other horses, I would go and look at them. If I could not comfortably afford the cost or the time, I would pass on them. If this involves selling horses to have time or room, I would definitely sell the horses I already have before I bought any.
That said, I would be prepared to drive them and spend time with them to see if they are "meant to be mine" horses. If they would fit in with my desires. You may get there and find them to be different than you envisioned.
Good luck with your decisions. Take your time and make sure they are right for you and that you can afford them and have time for them.
I have been to a few of Ronnie's auctions. The driving horses I have seen there, have been driving around before the auction so you can ask questions if you are interested in them. Also the people you meet there may have others that they would consider selling. It is a great place to meet other mini owners and mini sellers and trainers.