I used just a bag of builders sand i think it was from bunnings, you just need to give it a real good wash!!!
Clown loaches are slow growing from what i have read and IME i have had my 6 for nearly a year and they have grown a bit but they are far off out growing the tank, then you could always sell or swap them for smaller ones
as for the bamboo i don't think coating it with anything would be a very good idea, it would probably take awhile to get to the rotting stage and then you could just replace it maybe???
As for the gourami, you could alway get a selection?? Dwarfs are quite nice and come in pretty colours I personally like my golds and peb's my kissing (i can't see how people think there ugly )