Provided the tyres all meet or exceed the manufacturer's specification I can see no reason why it would not be legal to mix them. The only rule I can think of about having different tyres on the same axle is regarding cross-ply and radial, nothing to do with brand, rating etc.
Personally I'd want my tyres (per axle) to be exactly the same, which to my mind would mean buying the exact same tyre as the one you have, which would therefore have the same speed rating. But you should be fine to have them different if you wish.
It's already been mentioned, but when you say 'new', is it old but unused? I'd check it's not too old, it would be a shame to buy another tyre then have a problem with the one you have perishing from age shortly after being fitted.
