I believe it is safe for African players to play for small clubs in Spain. Although racism is an issue, I believe it is more of an issue for the fans. Off the field, the players generally respect each other.
It would also be almost impossible to tell African players that they are not allowed to play for small clubs in Spain, even if the idea is to protect them. Banning someone because of their race is racism, no matter the reason. The real issue that needs to be addressed is the amount of violence and racism perpetrated by and against the fans.