Can you put this on the bottom of the oven?

Yes, you can use this oven mat by placing it at the bottom of the oven

Do these smell when you turn on the oven?

I have not noticed any smell when I turn on the oven with these. I hope this helps you. I am very glad I made this purchase.

Does it change how the oven heats or cooking time ?

No it does not, just don't keep them in any that cooks over 475,and take them out to do a self clean if you do not they will burn right on to the oven.I know this cause I did a self clean and the burned right on to the oven base and you will not get the stuff off.