New to engineer
FT is meh DPS wise, and bringing it into a dungeon might no be the best choice because of its relatively short range. It tends to miss a lot too. Grenades can do a lot more damage especially if you have incendiary powder to add burning or precise sights to add vulnerability. Also, grenades have a longer range to allow you to stay out of danger.
Sigils’ effects work whenever you swap, so from both p/p to kit, and kit to kit.