Yeah I guess I should have qualified my statement with"unless you know what you're doing and have a plan", didn't mean it was an absolute rule, just that you need to be aware of what you're doing if you're doing mixing and matching. And in general, you're probably better off with bias-bias or radial-radial, in my opinion.
The point was really that speed rating or load bearing numbers can definitely be mixed and matched as long as they suffice for the vehicle per individual tire.