What type of shrimp are they?
Wood will release tannins which will slowly soften the water and lower pH. Not generally a serious concern, just boil any wood you'd like to add for many hours, changing the water frequently. This will help reduce tannins, but I don't think you can ever totally eliminate them.
Stone can be an issue if you are collecting it yourself and don't know what is in it. Many types of stone will leach different things, but it's a slow leach in the more neutral pH range. If you use CO2 or have more acidic water, the leaching process is sped up. Buy stones from a reputable vendor to be on the safe side.