Get a 55 gallon LONG and split it into approx 5 gallon sections. That will house 11 of your males. Then another 20 gallon LONG can be split into 4 sections of approx 5 gallons each, which will house the remaining 4 males .
You could run an external filter in each tank, just section off a small partition in the centre of each tank, approx 3-4 inches wide , and into that section you can put your filter intake pipe . By containing it in it's own section you won't have the risk of any of the bettas being sucked against the intake. If you don't want the pipe to be blatantly visible when you look at the tank, just place a plant at the front to block the view somewhat. The same goes for the 20 gallon.
To stop a strong outflow disturbing the bettas, you can simply attach a spray bar and point it ( the holes ) at the back wall of the tank to deflect the flow of the water down there instead of straight out. Also add broad leaved plants at the back to deflect the flow even more.
Eheim external filters are very good in my experience, I would reccomend them as they are easy to operate and easy to service. And you don't have to buy any brand new ones . Check the pet sections of the classifieds , or Ebay for second hand filters.
As for heating, you simply cut the dividers carefully to allow a heater though, but not be so big that bettas can wiggle through . The plastic of the dividers should fit closely to the heater but not actually touch it. Leave a 2 mm gap between them.
It would be cheaper and less complicated in the long run to have two larger tanks divided and externally filtered with one big filter each, than to have all your separate tanks everywhere , or several individual sponge filters and air pumps to run them all if in a large divided tank.
