ok, the brick stuff is probably eco-earth or another brands equivalent. I would keep a close eye on the aspen for mold.

I would strongly suggest that you get a thermostat for the heating pad.

Here are 2 good thermostats. I would stay away from the thermostats sold in pet stores, they are overpriced and not the best.

The hydrofarm thermostat runs about $30 and will do what it needs to, but they aren't very accurate compared to better thermostats.

http://www.amazon.com/Hydrofarm-MTPR.../dp/B000NZZG3S

The Herpstat is the best you can get, they do have a higher price tag though... herpstats run $100+, but are certainly worth it (these are what I use) they are safer, and much more accurate. they also have more capabilities than your standard thermostat.

http://spyderrobotics.com/

The dimmer will work but if the temperature in the room changes the dimmer will have to be adjusted to maintain the proper temperature.

For heating pads to be effective the substrate layer should be no thicker than 1/2"