Query regarding amex mrcc

I'm new to amex so I will be asking stupid questions

1. Do we have to do any kind of enrollment for a 1500x4 milestone like 20k milestone and 20k milestone enrollment is one time?

2. Do we have to do any enrollment for an 18k and 24k gold collection?

3. If I get another amex card will points of both cards get pooled together and do we have to do something for that?

4. Any tricks to achieve milestones?

5. What are other useful offers/benefits of amex?

Thanks in advance