I think all of the manufacturers would say only mix their paints to achieve colours, but I personally would use whatever I have to hand.
I use and demonstrate SAA Watercolours around the local art groups (Wigan / Warrington) but if I have bits of older colours around, I will use them as part of a mixture.
The one drawback to this is that when you come to mix the same colour again, you may not have any of a certain colour you've used before, so be careful of that.
But I think in general, Watercolour will mix with Watercolour.
I hope the above helps, but come back to me if you need more information.
PA and RSC Wigan / Warrington / St Helens