Great Stuff is actually waterfall foam or foaming crack filler and designed for outside use.
As for silicone, it just needs to be labeled 100 percent silicone and not have added mildew preventive in it.
Some 100 percent silicone products have a lingering smell that takes time to disappear and some the smell clears up faster. IS: GE's version 1 lingers and GE's version II disappears in no time at all.
I don't know what an evergreen bromeliad is but depending on the size of your tank, any bromeliad can be used as long as it fits and has been rinsed of all fertilizers and pesticides.