You wouldn't be able to trigger the start of recording on the cameras (so that they all start recording at the same time) via the record button on ATEM Mini ISO, since the record start/stop flag will not pass through the SDI to HDMI converters.
Also, with the Blackmagic Pocket Cinema Cameras, matching timecode information is sent over HDMI to each camera and this is stamped into each in-camera recording to allow for matching up the timelines in Resolve.
The timecode information won't pass through the SDI to HDMI converters, so you will need to separately feed timecode information into each camera. And the problem then will be that this externally generated timecode may not match the timecode used for the H.264 program and ISO recordings on the ATEM Mini ISO (unless you have a way to extract the LTC timecode from the ATEM Mini HDMI output and send it to the cameras). So you may have trouble matching up the in-camera recordings with the ATEM ISO recordings on the timeline in Resolve.
Obviously the camera control signals won't pass through the SDI to HDMI converters either, so you won't be able to use the color correction and other camera control features in the ATEM Software Control panel to remotely control the camera settings on the URSA Mini Pro.
Finally, there was a firmware update released for the Blackmagic Pocket Cinema Cameras which supposedly adds improved metadata support for the ATEM Mini Pro ISO:
https://www.blackmagicdesign.com/suppor ... 4498a004c3I'm not exactly sure what these metadata improvements were exactly and how Resolve might use them, but the URSA Mini Pro likely would not have this metadata information in its internally recorded files.
If you are ok with all of these restrictions, or if you just want to use the H.264 ISO recordings from the ATEM Mini Pro itself to re-cut the production in Resolve, then I'd guess that would probably work.