pin 5 is just 12v power in.
Brake and clutch switches also have a 12v feed to them (with their other side connected back to the ECU), so its just a useful place to take it from.
If you needed to add a clutch switch, it would have 12v to one side of it, and the other side connected to the ecu.
Also, your saying you'll connect these to the "white ECU connector". The white "T15" connector is actually in the plenum, behind the ecu, not on the ECU itself. If there is a connection between that white plug, and the ECU harness, then you can simply attach them there as you describe. If there isnt a connection there, you need to take it directly to the ECU multiplug itself on the appropriate pin numbers.
I've never done this personally, so i dont know whats present and what isnt.