The marine true plant is mangrove. You are probably thinking of macroalgae which is not a true plant (Caulerpa spp, Chaetomorpha, etc.). This is what people usually have in their refugium. Caulerpa is probably the safest since under certain conditions Caulerpa can reproduce sexually and poison the tank.
A sugar gain size sand is ideal. I prefer black so I use Estes' Ultra Reef, but if you want all white go with a calcium based sand.
Most gobies will prevent the sand from staying live in a 29. I wouldn't recommend one to keep the sand clean in that size tank, there are better methods.
Ideally the fuge is not in the tank. This takes up valuable display space. A HOB
or better yet a sump is a much better option.
Banggais can be in groups. They will actually host in anemones just like clowns.
Ocellaris can mature very young. Naturally there are many in one anemone. The largest will be female, the second largest male, and the rest immature. When the male or female die the next one in line in size will mature to take over that position. There is not a minimum size for them to mature at, it simply takes times. It takes approximately one month for them to go from one stage to the next (immature to male, male to female). So one that has been alone for two months is female and one alone for one month will be a male. Ideally get two that were together and getting along at the shop and you should end up with a nice pair.