Yeah, you could get away with a wide breadth of rods on Slate. It's all personal preference, and there's different spots where you'd probably ideally want different rods at times. I use a 7'6 rod there as a compromise FWIW, but that's because I don't have anything between that and 9'0 in my current lineup. I feel like an 8'0 4 or 5 wt would be my perfect choice, but I don't own one of those. ;-)
Agree with FI. Take a 9'0 and something a little shorter and decide when you get there.