Yes, you can use any fixture for the light. The only thing "special" about the ones at the pet store is that it has a hood to direct the light downward, but as long as you follow directions for how close the light needs to be, it doesn't really matter.
I actually recently used a cheap under...
Where and how are you measuring the temperature? Heat mats are not supposed to heat the air inside the tub, they warm the bottom, creating a "hotspot". If the roaches are too cool, they'll move to the bottom; too warm and they'll move up.
No, unlike some meds that are refrigerated to make it last longer, metronidazole (Flagyl) MUST be refrigerated if it is in liquid form. My knowledge comes from being a vet tech, :)