Regarding RCM Quary

As per Notification 13/2017 CT (R) RCM is applicable on GTA Services & Recepient is the person who is liable to pay freight.
And as per GTA definition provided under clause (ze) of Notification 12/2017 CT rate any person who provide services in realation to transport goods by road & issue CONSIGNMENT NOTE (GR) .

So the condition attract RCM :
1. Must be GTA
2. Issue G.R

Now come to your 2nd leg of Query : GTA are not required to be registered unless his services Under RCM (Refer Notification 5/2017 CT) , So even though the GTA is Un-Registered , the prescribed Recepient is liable to do RCM .

GTA Services are not liable to Reverse Charge :
1. Supply to URD individual
2. when GTA pay Tax on FCM @ 12%
3. when provided to Govt ,which has taken registration on for the purpose of deducting TDS.