With a normal salinity and alk/Ca/Mg maintained at reasonable levels, they grow and grow and grow . . . I am cutting one of my leathers into frags and will be trimming a second. Really nice ones don't grow as fast (Tyree green polyped leather, Fiji yellow leather). Xenia has a reputation for taking over tanks with runners and new growth; my Cespitularia (similar) will do that but is reasonably easy to control. Xenia/cespitularia can be dependent on some trace elements; mine was growing, then died back after a period of no trace replacement, and is now back to growing with correction of general trace elements.
Zoanthids - growth rates vary highly; some uglier ones grow like weeds and need to be pruned. Watch for palytoxin, so be careful when fragging.
Polyps - green star polyps are reputed to be very weedlike and fast growing; bright orange and yellow clove polyps grow less quickly and are cool.
Mushroom corals can be brightly colored and are pretty easy as well, except for Ricordea Yuma which bleaches with almost any UV unless VERY slowly acclimated (and I don't know how high they can be acclimated).
I love my softies/zoas/shrooms (and a Yuma in the sump)! Be prepared to prune them, which is easy once you get over "I'm cutting a coral branch off with a razor blade or scissors!"
