Do rechargeable batteries have the same mAh?

This will work. You can use it. The smaller one may be damaged before long though

The smaller one will run down first, so the extra capacity of the other one won't give any benefit in electronics, were the device tends to shut down at some predetermined terminal voltage. If you do this in something dumber like a flashlight, you will see a noticeable dimming as the smaller one's terminal voltage drops, and at that point you should stop using the battery until it (both cells here) can be recharged. it depends on how connected is series sure except the small ma/hour will determine longevity in parallel that is a very bad idea the weaker one will tend o drag the healthy one down even to the point of failure. Even if both are the same ma/h do not tie them in parallel