When you are using ACES in DaVinci Resolve, your work in Fusion will automatically take place in linear space, but there are two points to be aware of.
The good news is, your workflow should work just out of the box. Your footage will be in linear space (ACEScg) in Fusion and automatically come back to the ACEScc or ACEScct for grading.
However, the Fusion View LUT can cause confusion. It defaults to an sRGB output which is not aligned with the ACES Output Transform. Meaning your footage will look different in Fusion compared to the edit and color page.
A simple workaround is to disable the view LUT altogether and attach an ACES transform node to your flow. If you need more flexibility, you can install OCIO and add an OCIO view LUT instead of the Managed view LUT.
Here’s the tutorial:
You may also be interested in these tutorials about linear workflow: