Okay, so it's been a LONG time since I used heat mats and I need some help with one.

I've a ZooMed 16w heat mat; approx. 12". I plan on using it for my Ball Python's 10 GAL.

Where should I place it exactly? His hide is on one side and his water dish on the other.

just stick it on one side of the cage, covenring about 1 third. under the tank.

at least, I think that's what your supposed to do.

Put it on one end. Make sure there is a hide on both ends so the snake can choose whether to be warm or cool and still feel secure. The water bowl can be placed whereever. However, if you place it on the warm end, it will evaporate faster

Put it under, not on the side.
Under a hide.

Okay, but I don't have room for another hide b/c I spoiled him with a big water dish ...

I've one of those thermostat thingies ( ZooMed ReptiTemp 500R Pro Series ) and it has two plugs; one for the lamp and one for the mat. Would it be alright hooking them both up as long as the thermostat isn't set too high ?

well ball pythons should have warm water so maybe rearranging could be done to warm the hide and water or getter a bigger or another heat source
remember to keep an air gap between the mat and glass or it can overheat, if you are putting it underneath that is
no idea about the thermostat i don't use them

Those Rheostats are usually rated for a certain combined wattage of heating accessories. Check the labels and if your mat and lamp are at or under what the thermostat is rated, you can do it. Just make sure you get the temps right before you put the snake in.

Question ...

I know I'm supposed to use the little feet that came with the heat mat, but the 10 GAL hangs over the edge by ~2-3". Would that be sufficient ventilation for the mat ?

well on one of my sidewinder tanks it hangs over the edge a little and i saved the little feet just incase other ones need it. i think it should be enough ventilation

I prop my corn's tank up with a couple flat pieces of wood, one under each end of the tank. This also lets me slide the tank over the edge to access the lid without tilting it.

a ball python needs at least a 20 gallon tank, 30 would be best and could be used its entire life. You should invest in one of those asap.
The heat mat should cover one third of the tank, on the bottom.
hide on each side.
a 16 watt mat shouldnt need a rheostat, but test it out and see if it gets too hot. i use a zoo med 500 thermostat for my BCI and it controls the temperature withing 2 tenths of a degree really well, a great investment i would say.
there will not be enough room on the bottom of the 10 gal for the mat, it will cover too much of the surface area and could bake your snake.
All the best.

any follow up on you ball python?
how is his setup now?

I'm not a big fan of heat mats. I prefer overheat heating, I think it simulates the sun a little better. I also had a mat go haywire on me and heated the tank up pretty bad.