Any leather will grow too large for a 6 gallon very quickly. It was only a matter of a few months for my cabbage, devil's hand, and finger leathers to be large enough that they wouldn't fit in your tank. All are the size of my hand or larger & started very small. Toadstools get so large that they won't even fit in my 55 LOL.
Snowflakes, GSP, & xenia will take over for sure & it's hard to have a dedicated rock in a 6 gallon. My GSP in one spot has reached the top of its rock & is reaching up beyond the rock, so they can spread by reaching across empty space (I'm watching mine to see what it will do LOL).
As Oodley said, the clove/snowflakes will grow around stuff, so those could be a good option once you get your other corals in place to fill in blank spots.
Ricordea mushrooms are great for small tanks. I had a 3 gallon with a few ricordeas and some red mushrooms, but the regular mushrooms can spread & get large during the day, but not too large for a 6 gallon (they looked huge in a 3 LOL).
We have an encrusting psammacora that's doing great in our softy tank, but I don't know how aggressive they are in their encrusting. You could also try a small trumpet coral frag even though it's LPS. Our two seem to be pretty slow growers & have stayed compact in the year we've had them. You could also try a very small chalice frag, ours isn't spreading very fast. Our hammer, galaxea, & pagoda are growing pretty fast, so wouldn't recommend those.
Here are some
nano coral recommendations, but keep in mind "nano" is like under 55 gallons or something like that.
You could look through the Nano forum here for similar tank size builds for other ideas, too.
