Hi rose-anne,
All images are RGB. Which flavour of RGB do you mean? https://www.damiensymonds.net/2010/11/colour-modes-vs-colour-spaces.html
No, there is no benefit to doing so. It would only chew up your time.
Cropping should not be necessary. You would simply resize.
No, I can't think of any reason why it would make any difference.
Your description all sounds right to me. I'm curious what you meant, though, by "combining two images". Do you mean a photo, plus the text?
Will these prints be for commercial sale? Are you printing them on your own printer, or through a lab?