No, your ADA will remain delegated till your next action.
You can change the pool anytime, you don’t need to stop the delegation process; but if you will want to stop the delegation you must go to REWARDS - press WITHDRAW and deregister your address (you should know that you will still receive rewards for the next 2 epochs)
There is no good pool, all pools return rewards; on long-term you will receive the same % reward (around 5%) doesn’t matter if you will delegate with a small or big pool. The only difference is that big pools create blocks more frequently and you will receive small rewards each epoch while small pools will create few blocks and you will receive big rewards but once at few epochs… any way… you should know that on long-term (1 year) no pool (big or small) can’t give you more than ~5-5,5%
Got another question, because I have delegated my ADA, I want to transfer them to the exchange, but when I am using send and puting password - there is some error: Error received from server while sending transaction. What now?