What happens if I put two batteries in series with different amperage?
The terminal voltage of the two batteries in series will be 7.4V.  Assuming a continuous discharge, you will pull slightly less than 850mAh out of the series string, at which point the 1000mAh cell will force the 850mAh cell into reverse bias.  The result will be at best a forever dead 850mAh cell and more likely leakage or possibly real mayhem like fire or explosion.  

Long story short, when making a series pack, use cells of the same capacity, chemistry, and part number and use a battery management system to ensure that individual cells stay within normal limits.