Kings is big but it can be a bit quiet at times. The other big FC that I see a lot of is
Artemis Moonlight, there's an ex-GAFfer in that one. I see sprouts in Choco Bros, Crystal Chronicles, Children of Light, and Heroes of Eorzea too if you're looking to do stuff with other new players. They've all got 100+ characters which seems to be above average compared to most.
The FC situation on Cerberus in general feels odd coming from Ultros. There are so many level 60 characters with full 210 gear plus an Anima who aren't in an FC, I don't think I ever saw that on Ultros. I suppose you don't really
need an FC. I didn't mind being on my own for a while but I like seeing people talking even if I don't join in much (plus the FC buffs are nice too.)