Are there build suggestions there? I have looked but knowing what polarity to change to is tough. I may feel te skill I want to add is good .... When it's not really. Right now I am just leveling things to 30.
You are thinking way too much about this. I'm at Mastery 8 and I've only forma'd one of my items. It's helpful, but not critical. Just do what you think would be fun. Experiment. A lot of what makes this game great is the flexibility it gives you as a player.
On a rifle, you can never go wrong putting a V in there with the forma, so when you have a high level Serration, its cost won't be the 11 or so points it normally would be. There's my one suggestion, if you really want some guidance.