Hi Gaz,
I can supply the gasket you require, I have this in stock.
Regarding the studs I can supply these individually at £3.75 or £45 for 12. I do not supply nuts for these. I have used auto imparts in Leighton buzzard for these in the past. They will supply taper or radius seat with a 19mm hex.
As regards to mix and matching I am no expert but the only consideration I can think of is the weight difference of one locking bolt verses a stud and nut affecting balance?