Is the Emma Original Pro too firm for a side sleeper who weighs about 9 stone?

For a lighter side sleeper, probably yes. The Pro is medium-firm with zoned pocket springs under the foam, and at around 9 stone you will not sink far enough into the comfort layer for the shoulder and hip to be cushioned, so you may wake with a sore shoulder. The Emma Original, which is a true medium with a memory foam layer, suits lighter side sleepers better. The Pro comes into its own for back and front sleepers, for anyone over about 14 stone, and for people with lower back pain, because the firmer support keeps the spine level. If you do buy the Pro, the 200-night trial means you can swap if it is wrong.