Mixing chains

Mixing chains is fine as long as one isn't significantly more worn then the other. I ran a sram/shimano chain on my mountainbike made up of the excess from about 10 new 9spd chains.

Wouldnt you need loads of them shimano joining links?

Nah you don't actually need to use those joining pins you can use the old ones if you don't push them right through. Of course shimano wouldn't tell you that and to be honest it is quite difficult getting them pushed out just far enough to open the link without making them fall out.