Service Monitor and Activity Monitor are the two heavy hitters. They both write to a TSDB on the local FS in a directory that you specify in the configs.
I would split as I mentioned above:
Host1: CM, DB, Amon, Rman
Host2: Smon, Hmon, Event, Alert
Service Monitor - resource usage dependent on the number of services being ran. It will grow as new nodes are added, as the new nodes will be running new services.
Activity Monitor - Collection information on MR.
Host monitor - Resource req. will depend on the number of hosts; will grow as new hosts are added.
Reports Manager - Is all about the reports. This is usually light if only the pre-built reports are in place and generated. I have had to bump this up as the cluster usage and size grew. I haven't looked into it but it is likely because the reports run on a schedule even if they are not used.
Event Server - Events can be generate out of any metrics or log entries. You can add more or more can be generated if they are triggered by changes to the system.
Alert Publisher - Resource req. will depend on the number of alerts. This will grow with new hosts and services. More alerts can be added.