There is no way to do what you want by just using one inputs.conf file. The closest I could think of would be to have one primary inputs.conf app that defines all the monitoring stanzas and disables them, and then a set of secondary inputs.conf apps, each assigned to just one forwarder, that just enables the appropriate stanzas for that forwarder. But I don't recommend doing that, as it's just another layer of complexity to manage (so now when you change something you have to change it in at least two places instead of one).
In the past, when I've had the need for very complex inputs.conf configurations, with hosts collecting data on behalf of thousands of other hosts, I've had a lot of success with building a spreadsheet that tracked all my inputs and then a Python script that processed the spreadsheet into the inputs.conf file(s) I needed. This had major benefits: I was just editing things in one place (the spreadsheet), which was a lot easier to read, sort, and search than an inputs.conf file; if I needed to change the way things were broken up, I could make the change in the Python script and have it automate the changes downstream; and much lower typo risk. I highly recommend this approach.
