2 rats won't feel lonely or lost in a large cage. They will enjoy all the room to explore. I don't have an answer for the nails, but I am going to replace a ladder with lava ledges and see if that works.
I once had a mischief that grew to 13 rats in a DCN with extra shelves. Was 13 rats too many? Yes, because I didn't manage to have the one on one time with each that I would have preferred to have. It was especially unfair in my view to Xena who was my first rat and the one that I had a unique bond with, but didn't spend the time I would have liked with, to Gabrielle who was uniquely bonded to me but didn't get the amount of my time she should have been entitled to, and to Apollo who was one rat from an "incidental" litter (my sister adopted a rat and litter) that bonded to me but had to be adopted out because I was unable to keep "just one more". Was 13 rats too many? No, because each was a rescue, they all knew they were loved, and I did make them as happy as I was able for the time I had them. Would I do it again? No, I want to have a smaller number of rats probably not more than 3, certainly no more than 5, so that I can give each as much of my time as possible for all our well being.