Oh, and before you worry too much, the heat mat is not specifically "under tank", it just can be used that way. However, it's originally for desert reptiles, so I imagine it gets over 30C. Even then, it will only occupy one third of the vivarium, so the snake is free to cool down elsewhere.
Also, you guys might have misunderstood me or I perhaps wrote unclearly. I only have one snake right now, a cali king. In the past 7 months, I've had a corn and a yellow rat too. Relax, they're perfectly fine! They're just taken care of by others now.