Yes you can do this unless it has a timed charge cycle, in which case it will not get the battery fully charged if you wait till it's running slow. If it instead just trickle charges over several hours, that will work fine. You can measure to determine which it is if the specs don't elaborate. It could even have a proper NiMH charging IC that senses Delta -V, but I doubt it unless a fairly expensive shaver.
Also keep in mind that if the original batteries were especially low capacity due to being designed for high current instead, the performance of the shaver may suffer from a slower running motor.
What cell size is in the shaver? If they are AAA, I would not get 1500mAh rated AAA because it's probably a low quality cell with a dubious rating or at least very high self discharge rate. If they are AA, not only would 750mAh be low, but even 1500mAh would be very low for that size. If they are Sub-C, even more so the capacity seems too low. Are they some fractional sub-C size? That would make the most sense.