Quints cover 1 engine and 1 ladder requirement for any mission. So if you have a mission that requires 2 engines and 1 ladder, you can send 1 engine and 1 quint because it covers the 2 engines and the 1 ladder requirement. If you want to dispatch quint as a different class then go to the vehicle settings, make the custom class "quint", and select "only dispatch this vehicle as its custom class" meaning that it wont dispatch as an engine or a ladder and will only dispatch as "quint". Then go to your ARR settings and make one that has the custom class "quint" selected for however many you want.
The way that I have my ARR set up is by having an ARR for each unit i use. So I have an ARR called engine that starts the closest engine, one called ladder that starts the closest ladder, etc. I also use LSSM so I can select the same ARR multiple times in order to dispatch an entire mission right from the ARRs.