A:AnswerYes, you have to use the Alpine dash camera to use the screen.
I have the Nextbase dash camera and it does not connect to the Alpine head unit.
A:AnswerIf you use the Apline dash cam and Alpine rear camera you can connect one more factory camera. However, I could not my Nextbase 622GW with the rear camera to the Apline.
A:AnswerIt fits a single din slot. The 11" screen is large, but very nice. I suggest you get the overall dimensions of the screen to see if it blocks any a/c vents or anything. It fits great in my 2015 Sequoia
A:AnswerI have the factory installed back up camera on my Toyota Sequoia and it did connect to the Alpine unit.
What I found does not connect are dash cameras that also come with rear facing cameras that are not Alpine.