I think the dark grey shirt with the black vest may be a bit dark, but I have seen uniform shirts in light grey which might be just far enough away from white that it would set you apart. Another thought would be to go with tan/khaki shirts. I think any neutral color would go with that tartan and the tan might draw out the yellow stripe a bit.

I second the idea about yellow flashes, although I would keep them on the green hose, or even move to blue hose. You could also match the ties to either the hose or the flashes without too much difficulty. I think that black is a bit too cookie cutter, and perhaps a green tie behind the black vest might just stand out a bit more.

As for Sporrans, I say stay with what you have. They are versatile. To me, and I may be wrong in this, the horsehair does not look right without the doublet.