Early Scout, while still could be used in rock garden, might be too 'garden looking' for the classic rock garden.
Dwarf fernleaf, P. lithophila Kotov could be a great choice if you have enough patience waiting for 5-6 or more years untill it become 1x1' mature plant.
Another great short (only 1.5' tall) plant is P. veitchii, but at maturity it could be pretty wide, up to 4-5'.

My early scout is very wide, I would not consider it for the rock garden