Dan,
The nib will be marked on a Teal Blue, but then you have to take it apart to see the mark and I believe that if it ain't broke don't fix it.
As the pen is an Aero I assume by the colour, then you can get away with some of the saturated inks as long as you flush it well. I also keep some Parker Quink Washable Blue around as a fill of that does wonders for cleaning a "51" out. I have used PR Supershow in an Aero "51" but would not use it in a Vac.
If you don't want to gamble on the really saturated inks, then Diamine works well in a "51" too.
The chart of nib sizes is on Parker51.Com, but it relates to the early Vacumatic nibs I think. US Nib sizes run smaller than the English nibs, so a English M is nearly a US Broad.
There is a simple answer to this, try it and get a feel for it, see how it writes for you

The only "51" nib I could never get on with was a XF in a Vac that I cured by exchanging it for a M.
Enjoy the pen.
Jim