My view is that I wouldn't do it myself. I'm not going to say that the OP is a crazy death defying loser because he's changed one tyre and not the other though, because I don't have any source other qualify that advice.
You weren't helpful and put your point accross poorly without anything to back up the statement. You'd think that with your age/experience/wisdom/internet skillz, you'd be able to do better. You and others are exaggerating claims that because he's mixed his tyres, he's destined to wrap the car around a tree with no hope of recovery, when all that's really likely to happen is that the performance and the extreme end of the spectrum will be reduced. If the OP was Rover416i or some such other nutcase who raced everywhere then your point would be a lot more valid, but do you really feel that the OP will be affected that much by this? Or are you just jumping on the bandwagon?
I don't know exactly what you read, but I merely stated that mixing tyres on the same axle is a no no, which it is if you want stability when you need it most. How is my point less clear than someone who asks how good a tyre is when they don't even state its purpose/vehicle it is going on? with further information it would be a lot easier to state the exact details of how it would affect his cars performance.
I never stated any of the BS you have written above, so may I suggest, if you are going to patronise me at least do it with some degree of skill.
FACT: Mixing tyres on the same axle WILL affect stability in adverse conditions, it can also affect stability on uneven road surfaces. It will almost certainly produce unpredictable results under braking hard, ie, when you may need stability the most.
30 seconds on a search engine can yield the outcomes people have listed above. Sure if you are gonna be driving along a straight road in perfect conditions, they will prove to be adequate, but personally, I wouldnt be staking my life on it, and your contribution to the discussion is merely trying to rubbish me, rather than add anything constructive to the debate. The minimal affect of mixing tyres will be some reduced grip at the rear (assuming he has mixed them on the rear). The worst outcome is severe instability during acceleration (more pronounced torque steer) and a car that will veer heavily in some circumstances under braking (if fitted at the front on a FWD car).
Without more information in the OP about vehicle, placement, condition of other tyres etc, it is difficult to be more specific, so therefore, I limit myself to just staing its a no no.
As for whether you should mix makes etc of tyres, it was enough for Kwik Fit to tell my leasing company that the tyres should be swapped in pairs for them to agree to change 2 tyres on the front rather than 1, for them to spend the money and use 1 good tyre as an emergency spare, so I thought it would be reasonable for a competent tyre fitter to have warned the OP about the possible adverse outcomes of mixing tyres on the same axle. The fact that he didn't would suggest to me that as a tyre fitter, being held up as an expert in the matter, and better than a layman, I was justified belittling his expertise.
Now, rather than spend your time trying to bash someone who gave legitimate information, go spend some time doing some research and improving YOUR knowledge. A google of tyre bible is a good start, who knows you may even learn something on the interwebz eh!
If you are actually interested in finding out about tyre mixing, here is one link I found in a few seconds
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=136 with my google skillz
The third option, choosing alternative tires, should only be considered as a temporary solution in an emergency situation. Using alternative tires from different tire performance categories, with alternate sizes or different speed ratings can unbalance the vehicle's handling in poor weather or when pushed to the limit in an emergency.
Because tires play such an important role in every vehicle's comfort qualities and handling capabilities, it is always best to drive on tires that are identical in every detail, including tire brand, model, size and remaining tread depth. Anything else involves some type of compromise.