You can use drakx to uninstall it; I think it works even in terminal mode. Alternatively, you can use
Code:
rpm --erase packagename
You can install mysql alongside postgres without causing any problems, as they use completely different protocols and network ports.
There's a very good tutorial on getting started with postgresql at
http://www.postgresql.org/docs/. Basically you need to su to root in order to su to postgres to create a user account. Then you can create a database, and after that it's very easy to use.