When I make a new tree or add generations, people that I have tagged in Family Search photos do not always appear in the list with the photo. Plus I do not seem to get spouses of first cousins in my Rootsfinder tree.
That’s true. If you didn’t sync someone in a photo, they won’t appear in the list with the photo.
In order to get spouses of first cousins, you need to navigate to that cousin in RootsFinder, then click the FamilySearch icon in the toolbar, and select “Download more generations”. That will give you a chance to download their spouse (and children if you want).