Yes, you can. custom_homematic is, in contrast to the RaspberryMatic Add-On, NOT an Add-On. It’s the new version of the HomeMatic Integration for Home Assistant. The old Integration is what is documented here. custom_homematic will eventually replace the old integration, but is still in the testing phase.
Anyways, it’s the job of the integration to integrate a product logically into Home Assistant. The RaspberryMatic Add-On does NOT do this. Just by installing the Add-On Home Assistant doesn’t know anything about your devices. You always need an integration to make them work together.
This nomenclature is very important, so please memorize it. The RM-Add-On could be a different machine, virtualized, whatever. The Integration is then configured to interact with the Add-On (or separate machine or whatever). And this concept stays true for pretty much all Add-Ons and integrations.
If you install a Database-Add-On, then you still have to configure the recorder-integration to make use of it.
Finally: you don’t have to use the new HomeMatic integration. It’s just for early adopters who are willing to test things and accept things breaking due to rewrites of the code. If you want your system to be stable (until the new integration is released), stick to the old (official) integration.
Hope this answers your questions.