Hello I have customised my popgroups.ymt file to have a lot more variety in the traffic, Im very happy with it, however Im curious as to where I would add ambulances and fire trucks?

i.e <Name>VEH_TAXI</Name>

^^ Thats just used as an example....the cop cars have their own groups, so what would fire and ambulance come under?

I have tried adding ambulance to some of the groups but I haven't seen any in game...

Is there a special group I need to know about for those vehicles?

Just add an entirely new group, and then add it to the popcycle lines. The game may have hidden issues with doing certain things, so it's kind of a crap shoot whether something will work you intend it to.