There is no such thing as a "generic" electret mic. The store owner is just being lazy or got a box of parts that fell off a truck and doesn't know the model. While there is some commonality between electret mics, the voltage and impedance they want for best operation varies. The allowable range and recommended values are of couse in the datasheet. Electrets are cheap and available from many sources with the right documentation, so I would leave these on the shelf.
Since you've already bought them, you'll have to experiment. Start out with 10 kΩ to 3V and see what happens. This is unlikely to damage anything. If that drops more than a volt, use a lower resistor. I've seen some specified for 2 kΩ recommended operation. 10 kΩ is actually high, but it makes sense to start with conservative values and work your way down. Higher values may give you a bit more voltage out but also cut off high frequencies more.
My internet connection went down while typing this answer. When the submit seemed to hang I managed to copy the text in the edit window before it went away altogether. When things came back up, it appeared to have saved what I typed, so I just re-submitted it. I see now that the last two paragraphs didn't make it. Fortunately those were included in my manual save:
As for directionality, there is no way to know. Just try it.
A basic electret is a two-leaded device. Some have a FET or something
else built in, so have a power, ground, and output lead. Those can vary
considerably in how you are supposed to handle them. If you have 3 leaded
versions I would go back to the store and rerturn them unless the owner
can supply the proper datasheet. It sounds shady and a bad place to buy