Is it the "Node Status Polling" or "Collect Statistics Every..." which determines how frequently Custom MIBs are polled? Regardless, I'm thinking there needs to be a third item for "Custom MIB Polling" that can be configured independently. I may want to collect Interface Statistics ever 5 minutes, but I want to poll my Custom MIBs ever 30 seconds, etc.
Assuming both are collected through the "Collect Statistics Every..." option, I don't want to fill up my database with Interface Stats gathered every 60 seconds, which is the minimum value allowed.