>Stop the drugs
Most definitely not. You will condemn your Frenchie to even more seizures. That would not be fair. And, in all fairness, under the Animal Welfare Act you are required to provide for the welfare of your pets.
Bringing home a new puppy, you would expect an older dog to put a puppy in its place. Which is totally natural. Your new pup has to learn what is, and what isn't tolerated. Interfering would just confuse the issue.
>vet says there was no connection..
I've had a dog on phenobarb for 3 years. Like your dog, late onset epilepsy. She had her first seizure at 6 years old. At 9 years she is doing well on them. Has your vet told you that your dog will need blood tests regularly to measure the levels of phenobarb, and altering the medication as required.
I am wondering if you had not interfered in the "pecking order" that things would have been fine. I would train the pup to get used to a crate for time out whenever it gets over the top and too much for your Frenchie. I have to say that my bitch had no changes in her behaviour at all.