What are you using to check the temps? If it's an analog thermometer it would be better to get a digital one, or even better yet, get a temp gun. If the temp is really at 70-75 then it should be okay to leave the lamp on, but lamps tend to get really hot and I'd suspect the temp is actually higher than that.
If in doubt, turn all heating off until you have a reliable way of measuring and regulating it. I don't suppose the people at Petsmart told you that you'd need a thermostat? Pretty much any heating device you can buy can overheat if you don't have a thermostat to regulate the temperature.
A 10 gal is fine for a baby. When you can upgrade depends on the snake. A lot of people don't realize that ball pythons feel safe in dark, enclosed spaces, so more room isn't necessarily better. An insecure hatchling may go off feed if put in too large a tank with too little cover. If you want to buy a larger tank you can, but be aware that you'll have to clutter it up with hides and decorations, and even then the snake may very well refuse to eat unless you put it back in a smaller enclosure.
I can't really say for sure because most of it depends on the personality of the snake. Some of mine do fine in wide open spaces, others won't eat unless they have a nice, cozy tub to chill in.
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