The Top Fin 40 is probably way too much jetting flow for the girls. It'd need quite a bit of baffling and the intake strength is high enough to harm any girls who perch on the strainer. The Top Fins aren't "bad" per se, they're just not well suited to betta. The main problem with a Top Fin is it is NOT a biological filter. I'd advise any combination of Aquaclear filters over the Top Fin and would myself prefer using an Aqua-Tech filter from Walmart instead of the Top Fin.
The 40, even with heavy biological additions, certainly isn't going to be enough to keep up with your stocking plans.
You've got about 38 to 45 inches of fish planned there in a twenty gallon tank. You'll be making around at least 20ppm nitrate per day at minimum.
At maximum you may end up producing 60 to 80 per day. Additionally you're dealing with overloaded filtration while the bottom develops and/or any stalls and reversals in the bacterial in the substrate. Heavily overloaded tanks with poor filtration equipment can go wrong very very quickly. For this kind of load you're looking at using not only additional biological capacity but I'd advise getting one or two "Tripple Flow" box filters filled with Seachem Matrix (http://www.seachem.com/Products/prod...es/Matrix.html
) under a hand-cut layer of filter material.
To maintain these levels of over-loading I use fairly hard core equipment and material:
My 15R sorority (20 girls, five fancy guppy, one black molly, 60 trumpet snails) uses:
Fluval 105 loaded with Fluval bio-nodes, ceramic rings and a spray-bar return running at about 40gph return
Lee's Tripple Flow Box filter with 1/2" bio-sponge and 3/4 liter De*nitrate (seachem)
My 10R Tetra community (30 tetra, 10 fancy guppy, one chinese algae eater) uses:
Fluval 206 loaded with Fluval bio-nodes, micro ceramic rings, fine filtration card, bio-pro 30 Marineland Bio-wheel return.
Established three year old NPT bottom consisting of inch of dirt, 3/4 inch of aragonite and light dusting of rock. 60% coverage by plant.
24 hour lighting on a rotating position.
The amount of nitrate produced per day in the 15R is dropping steadily because of the box filter.
The amount of nitrate produced per day in the 10R is 35. This tank has $200.00 of filtration and more De*nitrate than the 15R.
Before I start advising special solutions what are your plans as far as planting and ornaments?