That would be way too many fish for the volume of water - Yes you can stock much lighter and will do fine. After your system is cycled you can supplement with something like blood and bone meal which will breakdown slowly (but you might not need to anyway). Aside from the filtration recommendation I think the other recommendation is 1 lb of fish per 2.5 to 5 gallons of water
http://www.backyardaquaponics.com/forum/viewtopic.php?f=12&t=6646
Personally I'd skip the towers for now and just use the grow beds. You could probably get this to work with guppies but I'm not sure what to suggest for feed or stocking density since most flake food isn't very good at providing what the plants need so we usually recommend a commercial pellet (flakes seem to be more geared toward maintenance and color but not growth of fish). There are plenty of options if you're going to heat the aquarium or you could, if it's legal use bluegill. Bluegill grow fairly slow but can be pellet trained and will eat enough to grow the plants pretty well.
As far as the filter - I can't see you needing another filter at this time. Your grow bed is almost 10 times the size of your tank in volume. You could go for years with a moderate stocking density before the grow bed needed to be cleaned. Using an RFF removes some of the solids that would otherwise breakdown in the grow bed and provide nutrients for the plants and I think you'll probably need those in this system. The RFF also adds maintenance and is probably just overkill in this case.
I don't think the difference between a 50 and 70 gallon tank would be enough to warrant up-sizing the tank in this case but I'm not sure
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