The 2 biggest problems I can see with pebbles are they would be a complete pain to clean and disinfect. The other being Im not sure how well they would transfer heat. I dont think it is really a question of whether they would harm the snake or not. Its more of would they really provide the proper environmental conditions for the snake. I like cypress mulch myself cause it really helps keep the humidity up.

As far as the heat rock, I myself personally would not use it. I just wouldnt want to take any chances of it harming my snake cause there have been plenty of cases of severe snake burns and even snake deaths from heat rocks. I dont care how nice or expensive the heat rock is I just wouldnt use it. Its ultimitaley your decision all I can do is give you my opinion.