I have the 500r and am currently using it. It just isn't up to the job an 8 slot rack holds IMO.... I honestly wouldn't trust it with a rack such as that. (This is why I am switching to a Herpstat for my rack.)

If you cut the flexwatt, then yes each cut piece needs to have it's own cord. You can get say 10 ft of flexwatt.. Wire that whole 10 ft and just weave the flexwatt from one shelf to another, or hook it up as back heat. I prefer each shelf having it's own individual cords just for added security... I can unplug any shelves I may not be using or put a dimmer if I happen to get another animal that needs lower heat for example.

Heard good things about the johnsons controls, it would be a good choice. Johnsons and Rancos are on/off thermostats while Helix and Herpstats are more like controlled dimmers.