Pool forming can be based on a few simple rules.  The goal is to limit the total number of pools and make stakes spread evenly among the pools. 1. Multiple pools can sign a block.  2. The maximum number of pool signatures a block can take is capped, for example at 20.  3. The reward coins a pool can get from one block is capped, for example at 5%.  4. The winning block is the one with the most stakes.  The miners can join pools freely and a miner gets his reward from the pool he joined.  Since each miner is after max reward, there will be 20 pools and each pool will have roughly 5% stakes at the end.

With the above rules the 20 pools will sign every block in the blockchain.  In the ideal situation each block gets 100% stake voting.  To prevent double-voting, each pool can only sign only one block at the same height.  In case of a fork, a pool can sign either branch but not both.  If a pool is caught signing 2 blocks at the same height the pool owner's stake is destroyed with the 2 signatures at the same height as evidence.