The big problem for making pool is shares check....
Example: If you mine 9-er chain and submit 6-er chains as share you'll get about 20 6-er chains in a hour.
Exactly! A miner properly optimized for short chains (to get the best possible reward from pool) would have low performance on 9+ chains. Sort of withholding attack from pool's point of view.
Well you can check if someone is getting a fast paced flow of 6-er chains and nothing else, then the pool put his share on old, until either the luck is coming back, or he is proved to be unfaithful.