Hi mate,
You shouldn't need to add anything else to the soil, you could add a little extra peat if you wanted to as the John innes does raise gh and ph even though it contains peat already but nutrient wise it's more than enough alone.
If you ever wanted to add to it at a later date you can just pop in a few root tabs like the tmc plugs if any plants showed a little nutrient deficiency or for really heavy root feeders
Sent from my iPhone using Tapatalk