As I did not see an option for it, here is my best guess as to how the new Pyro Mine will work:
Landmine and Extra Mine have no effect upon the Pyro Mine. By selecting Pyro Mine you will have a new mappable skill. You will only be able to place one Pyro Mine at given time and up to two normal Landmines in addition to it. This means that in total you can have up to three mines placed at any given time (2 Landmines and 1 Pyro).
Pyro Mine will subtract from your 2 placeable mines. 1 Engineer with 4 on the map is ridiculous, with 8 engineers on a team you could have 32 mines on the map at any one point in time. Stupid.
Besides all the other people posting hate for Brink I'm actually going to say I think that pyro mines will work like the turrets where you can only carry 1 type at a time.