Just put any champs in the spots. There is no difference if they are levelled or not, and it doesn't matter wich champs you use. The faction ring effect is allways the same.

To block less slots overall, duplicates of a champ you use anyway are welcome. Champion you use + 2 random rares in faction ring = 3 slots. Champion you use + copy of him in faction ring = 2 slots. Champions in the ring can still be used as normal.