I wouldn't recomend using a tank/cage topper for rats. Especially a 10 gallon, it's just too small of a base. To figure out how many rats will fit in your cage, go here:
http://www.rattycorner.com/odds/calc.shtml
I would strongly recomend martins cages for your rats, I know many people who use them, including me, and are very happy.
http://www.martinscages.com/
(I use the rat tower R-690 for my two rats. It holds up to three comfortably.)
Also - what ever cage you end up getting, it must have powder coated wires or the rat urine will seep through the metal and rust and hold oders very strongly.
Hope that helps!
-Sarah