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.
Optimizing a miner to search only short chains is kinda dumb. Let's say ALL the miners adopt it... suddenly no blocks are found and no one makes anything. Face it, why are 5-chains no longer accepted? Flooding the pool with 6 chains will give you a short term increase in XPM followed by either a lockout of 6 chains or a loss of the big dogs finding blocks who get disgusted funding all the small timers who will never find a block stealing all the XPM. The end result of both options is a subsequent decrease in XPM. Greed is always it's own reward.
Private short chain miner running on pool will increase mining profit at expense of other pool users.
Short chain miner released to the public will damage or kill pools, at least they will be forced to increase commission.
Right now scaring casual CPU miners off the pools will lead to more hashpower for other CPU-only coins.
Overall decrease of primecoin hashpower will lead to more profit for solominers. And the block reward for primecoin increases when difficulty decreases.
I see eldentyrell's trick of using signcrypton as the best way to run primecoin pool.
EDIT: link to tricone mining thread
https://bitcointalk.org/index.php?topic=49971.0