First Time w/Tri-Mix, what syringe to use??

So tonight will be my first time using the Tri-mix and I have heard amazing things about it.

My only question that I haven't been able to answer yet is the syringe size.

The compounding pharmacy gave me 29ga x 12mm syringes, and everything i see online (doctors videos) state that a 6mm is the best length needle to use.

I was able to find 31ga x 6mm at Wal-Mart, I am assuming that these are the standard insulin needles. So, any recommendations on which to use??

Thanks for the info, I hope that this fixes the problem oncev, and for all!