Are you asking if its ok to use the ET heat mat in the rack you could eventually have?

If yes, then I'd say no. You want to have a uniform heat source on a rack so it can be controlled with one tstat. Otherwise your temps could be all over the place from tub to tub.

I think you are asking if its ok to use the heat mat NOW...if you have a thermostat or dimmer controlling it, its ok. If you don't, then no, it's not safe.