md (software raid mechanism or watever u wanna call it) allows u to grow so if your raid is formatted with an fs that allows u to grow, then u r good to go (lvm will allow u to grow).
This was also my current belief, again thanks for confirming that.
you will not be able to add more physical drives to an existing RAID-5
You can since 2007 (mainline kernel 2.6.17) and therefore should be available in Centos 5.