what would be the reason to not request TCAS on planes <5.7 T or <19 pax?
No danger to airliners?
...I doubt that..
In my area of operation,except aerobatic planes,all planes I've flown or I know about have TCAS installed on them,if flown in controlled airspace. They usually select A2000 ,if flying VFR ,if my memory doesn't play tricks on me.