The only problem is going to come from the different tread patterns causing the van to drift a bit. It's not really unsafe at all, just might be irritating. Max PSI is just that...max. Your optimal PSI is stated either on the owner's manual or inside the driver's side door, or both. All the tires should be at the same pressure. A set of four tires that size at Discount Tire would probably run you about $450.