Peperomia Ruby Cascade needs well-draining potting soil. Because its leaves hold water, it is a good idea to avoid heavy or moisture-retentive soils. This will increase the risk of overwatering even if you don’t actually add too much water since the soil tends to hold on to the moisture.
In addition to good drainage, it needs well-aerated soil. Aeration and water go together because they need to balance one another out.
The reason is that there are only so many miniscule air pockets in the soil. So, too much water will clock up these spaces preventing air to pass through. Unfortunately, your plant’s roots need oxygen just as much as they need water. Without it, they end up rotting.
To achieve the soil you need, you can use a combination of 50% potting soil and 50% perlite, coarse sand or pumice.
Any of the latter three will work as they all work to improve drainage. But, they’re all different so, after a while you can decide for yourself which ones you prefer.