We had our older dog(a bitch) for 3 years before adding a pup(dog), and since then, 4 years ago, I have always walked both dogs together, it's natural to do so, given that not only does it "kill 2 birds with one stone" but I simply couldn't just take one but not the other, if you follow?
I believe in letting both my dogs off the lead once we are safely in the fields we visit. I keep them in sight and whistle to them if I want them to get back to me. Usually works! Dogs will naturally become defensive if still on a lead when encountering other dogs because they feel threatened.
Perhaps that could be the problem why the older dog, you say, is aggressive to other mutts? If and when that happens, you must let them see that you are firmly in charge, in other words that you are the alpha dog in their pack, because if you allow the dogs to rule you, they will take full advantage. You cannot allow your dogs to sense that you are weak, you must always be in control, and they must know that.
Firm but fair, never strike your dog, a firm tone of voice should do the trick, even if you feel slightly guilty at the time it has to be done. They need to have a pecking order and you must be at the top, and in your case, as mine, the older dog should be second and the sprog third. End of story.
The best of luck. Remember, spare the rod to spoil the child etc etc. Prepare and be ready for them to meet other mutts, but don't be afraid of it - dogs usually get on alright (most of the time) - a bit like humans!