RCM related matter

If GTA transpoter has charged gst @ 12% on his invoice, then no need to pay gst under RCM.
But if no gst has charged on invoice then you have to pay gst under RCM @ 5% irrespective of the fact that you have obtained regular registration or under composition scheme.
Also, since you have a registration under composition scheme then you will not be able to claim the ITC of gst which you have paid under RCM.

Yes..if You are liable

If the GTA is a Registered Person and opted to pay Tax under Forward Charge Mechanism @ 12%, then no need to pay GST on RCM basis.

But if not charged 12%, then you are liable to pay Tax @ 5% under Reverse Charge Mechanism ( RCM ) u/s 9(3) of the CGST Act 2017.

And since you are a Composition Scheme Dealer, you are not eligible to claim ITC on your inputs by virtue of Section 10(4) read with Section 17(5)(e) of the CGST Act 2017.

Warm Regards.