Don't let the snake free, it won't do anything to get rid of the mice anyway. Even a full grown ball python wouldn't put a dent in the population of mice. It may eat 2-5 a week, but giving an average mouse litter is around 8, there will still be more mice than the snake can eat.
Also, since you have already laid out poison, you have a higher chance of poisoning your snake. Any mouse that it eats could have eaten enough poison to kill it and the snake.
Best thing to do is get alot of mouse traps and set them all over your place where mice feces can be found. Check them once to twice a day and replace as needed. Seal any cracks, holes, openings and spaces large enough for a mouse to get through or chew around to make bigger. Use steel wool in places you can't seal with plaster, wood etc. If you see mice running around often enough, get a long strong stick and use it wack and poke mice to death. A corned mouse can be knocked unconscious or outright killed with a poke from a stick towards their head/neck area with about as much force you would use to break in pool or snooker.
Lastly, seal any grain based foods in tight plastic or glass containers. This includes all cerial, bread, pasta, oatmeal, crackers, cookies etc. Pretty much if it's not in the fridge/freezer or cans/glass, seal it. Keep the house clean as well. Don't leave uneaten food and dirty dishes lying around, vacuum/sweep up crumbs etc. If you live in an apartment complex, get your neighbours to take the same precautions and if all are diligent, eventually the mouse problem will be solved. You can also complain ot the land lord about the infestation.
Out of curiousity, why did the exterminator say he couldn't do anything?