Mollies are really weird in that they apparently can adapt to freshwater, brackish water, and full marine water 0_o
They do best in brackish water, cuz thats where they normally live in Mexico. Bumblebee gobies will also need brackish water...i think.
AFTER you cycle your tank, here are some stocking ideas i came up with:
#1. 8 corydoras of any variety (species that grow to 2", beware of emerald corydoras as they get big) 4 dwarf gourami of any variety (1 male and 3 females) 4 Pitbull plecos. and if youre growing more algae than your pleco can eat, get an amono shrimp or two.
#2. the 8 corydoras of any variety (or substitute with 3-4 yoyo loaches or botia loaches/other smaller varieties), 12-15 neon tetra (or any of the smaller tetra, or rasbora species, or two schools of different tetras. throw in either a few otocinclus or amano shrimp for even more clean up.
#3. 4 German blue rams, a school of some sort of tetra or rasbora...again.
One thing to remember that during breeding, the GBR could be agressive to bottom dwellers such as cories or loaches. however i would let a GBR owner decide on that.
Those are just some random choices that popped into my head, and some that i would've kept, had i had a change to redo my 40 gallon.
Obviously there are many more choices.