you have to consider swimming room not just gallons for dividing a tank. like in a 20 gallon, if you put 10 in there they'd each get 2 gallons, but a 20 gallon long is about 30 inches long, which means they'd have a 3 inch wide section each, which barely gives them room to turn
In a 10 gallon, you can divide up to 4 times with 2.5 gallons each- 10/2=5/2=2.5..It looks ok that way. If you can get a 20 long, that'd be pretty good, too.
10/3 gives enough space to roam- 4 a bit less, but not bad.
You can also try and get 5 gallon tanks, divided in half. It means more tanks, but you can also probably re-use your same heaters with a decent success rate as well as be able to not have them all in a big tank, so its easier to place them around the house.
^ even then, its very hard! I tried it, and it was flimsy.
Though, if you can manage to find a sturdy enough material, and perhaps double it up (say, craft mesh-type things, though these arent long enough), you can use the same spines as you would, with extra length. Its still hard, though, because one fish in the middle would be barely visible.
I posted this in another thread but I'll mention it here too just incase your missed it. Starting Sunday Petco is having a sale on their glass tanks. It's a dollar a gallon sale but it is only for 10 gallons and up. If your tight on cash this is the time to get a tank.