My questions: -do I really need to apply silicone before great stuff?
I don’t. I use the black waterfall/pond foam. I’ve never had issue w it detaching from glass.
-how much substrate should I use?(currently using 4.5 inch ABG, 3 inch rocks,which takes up half the volume of terrarium)
4.5 inches is a lot. A third of that should be adequate. I have one terrarium with clay balls and there’s two issues I think makes it a detriment to me.
1)weight. When I have to move it, the extra weight of that wet rock is significant.
2)volume. Because that rock occupies a significant portion of the space below the substrate, it can hold less water. It needs to be drained twice as often as the false bottom tanks.
-what type of plants?(ordering from NEHERP, thinking a Cryptanthus bivitattus)
Have fun with whatever you like. Pleurothallid orchids work well. Marcgravia are a fun genus.
-do I silicone cork bark on glass first or just place cork bark on greatstuff?
I foam the cork etc in place. It expands about 3-4x what you spray out.
-do I silicone the false bottom to the aquarium?
I use black silicone to obscure everything below the substrate so you can’t see the infrastructure down there. Many of us create a little platform out of the egg crate https://www.homedepot.com/p/Plaskoli...233A/202025149
then put some landscape cloth over the top of it
Create an opening you can easily access to siphon out water before you cover it all over with your ABG.