Check this thread out: https://community.smartthings.com/t/pre-release-google-assistant-relay-google-home-audio-notifications/114674
It’s got some nest camera integration (the whole project is in alpha though so subject to change). You could use that to issue start/stop nest stream commands rather than the built in Nest/Home integration.
Maybe something like this:
[IFTTT] IF - Google Assistant service - OK Google, show me a short clip from the garage camera
[IFTTT] THEN - Webhook service - Triggers webcore piston
[webcore piston] using assistant-relay device - Start nest stream from the garage
[webcore piston] Wait 30 seconds
[webcore piston] using assistant-relay device - Stop nest stream
[webcore piston] Harmony Device - Run “Turn on TV” Activity
I’m guessing that with Alexa’s IFTTT integration, this would be possible with either ecosystem (not sure if you need at least one Google Home to associate your nest cameras since I don’t have any…).