BridgePlayer One of my purchased apps is Textra, a wonderful SMS app. What I don't understand is if I install Textra via Google Play in my Google Stuff profile will it work while I'm in the Owner profile?
Depends on how you have to install the app and how the app has to verify the purchase. If the app is only available on the Play Store, you have to use the Play Store to install it. If the purchase has to be verified through the Play Store, you have to use the Play Store to verify the purchase.
If the app doesn't require Play at all, you could do it the other way around; setting up Google Play in the Owner profile, then use GrapheneOS' feature to push apps from Owner to secondary profiles.
BridgePlayer It is recommended to do it in a new profile.
Did someone tell you this? That Google stuff should be isolated in a separate secondary profile? That's entirely up to you. Dozens of threads by people asking the same question. Someone should probably make a community page with an article about it…