In answer to your questions:
1. Yes using the “Auto-fill” icon which is the first on the top toolbar. This will automatically add your most recently uploaded images.
2. Unfortunately not – when you insert cells it does resize the other cells. Note though that you can actually drag to the outer edge of the collage which doesn’t seem to affect the cells inside the collage. You can then build up new rows and columns there without affecting the existing collage.
The other thing you could consider is creating multiple collages and saving them, and then creating one mega-collage made up of those saved collages. If you do choose to do this, when creating the mega-collage you can change the dropdown in the Photo Basket (at the bottom of the editor) to “History” and directly drag on your saved collages.
– Regards, The Ribbet Team