I can't tell you much about ATOs except that I want/need one. My 90gallon can only go 2 days without me topping off so if I go away for long I could be in trouble.
As for shrimp, I added a skunk cleaner to my tank after 2 or 3 months. It did great, my guess is you will be fine. I also like the fire shrimp, I have one of each in my 29 reef. I don't think it is how many you need as much as how many you want within reason. They scavenge for food but they still have a little affect on the bio load.