There is a minimum of Ada you need to be a pool operator. Having the stake pool functioning, every other random person can delegate their Ada on those pools. The reward from the pools are well explicit, so you can calculate the expected revenue out of it. In terms of staking on Cardano, you will never be subject to unlock stake time on contrary to most other blockchains.. you can stake on a pool, and in the next minute you can swap for another pool. You only start earning after the next epoch, on Cardano those last 5 days. So you decided to stake on a specific pool, but the coins still remain on your wallet, but they are associated to the pool by a stake Key (works like a recipe). So if the coins are on your wallet, they are yours. The only thing to take into account is that the pool operator can change the fees and can stop creating blocks, that would mean you would no longer get rewarded.
