If the pool had 1 MS/s and some miner appears with 9 MS/s he will get 90% reward which is fair. No matter if the pool had no blocks for even couple of days before that.
Nobody could predict when the block is found (miners, pool owners, nobody). It is impossible to rent hashpower and be "on time" to find a block.
Don't worry, PPLNS system that is used in our pool prevents the pool hopping.