I too boil 14 gallons in a 15 gallon pot. I don't see an issue with your method. Fermcap is one way to stop boilovers but I'm not a fan and I've heard some negative things about it. With a careful eye and quick temperature adjustments, boilovers can usually be averted. I've topped off my fermenters with water just prior to pitching the yeast in the past as well, but made the adjustments in my brewing software ahead of time to compensate for the dilution.
Now I just keep a close eye on it until the first gallon or so is boiled off and then boil more vigorously.