What is the strategy mining pool servers use (regarding difficulty)?
It depends on the pool. Some pool use a fixed difficulty. Some use several and allow users to choose. Some automatically set the difficulty, with the aim that each miner generates a share every few seconds.

If the difficulty is too low, network traffic is too high (and some miners, such as the antminers, cannot generate more than 40-50 shares/s, regardless of the difficulty).

If the difficulty is too high, then miners generates shares too infrequently, and it may lead to a lost of efficiency, because the job changes every 10 minutes (each time a new block is mined).