Can I use evidences slots for additional letters of recommendations?
Yes you can but please note that the letter will count towards the limit of 10 documents. So ideally it should be brief a page or two.
I can share with you my own experience.
I got five letters of recommendation and was choosing among them. So I have chosen two the most authoritative people as my main recommenders and the other three letters added as supporting letters among 10 required documents.
I did not attach CV for signees of that letters and all of them were not longer than two-sided A4.
By the way, I have written a couple of other important aspects of the supporting docs - perhaps, they will be helpful for you too: