Hello Rodrigo
It is possible to configure a router to be both a mapping agent as well as a candidate RP. Simply configure both the ip pim send-rp-discovery command as well as the ip pim send-rp-announce command on the same device.
The mapping agent will select one RP per group. As stated in the lesson:
When two candidate RPs want to become RP for the same group(s) then the mapping agent will prefer the RP with the highest IP address.
In general, it doesn’t matter where the RP is. The RP is needed only to start new sessions, so the RP experiences little overhead from traffic flow or processing. However, the mapping agent should be placed centrally in order to facilitate RP elections, especially in a hub and spoke topology.
I hope this has been helpful!
Laz