The FH2E, DA312B16, and KBA525 are probably the best choices for your application. The DA312B16 and KBA525 have the smallest probe contact area of these choices. Information on these probes can be viewed at http://www.geinspectiontechnologies.com/download/products/ut/tg/GEIT-20022EN_thickness-gauge-probes.pdf
Maybe you should use specially designed probes for thin welds? I mean Tandem double transducer special probe for thin welds. These probes are successfully used for many years in Polish power industry. Contact me via email if you need some more informations...
To come back to the original question which asked for a standard for calibration of UT on small diameter piping.
As the GE instrument DMS 2 is referenced which is an ultrasonic thickness gauging instrument, and assuming that not a calibration block is required but written standards, I can recommend two European Standards (EN) and a guiding book on ultrasonic thickness measurement:
Non-destructive testing - Ultrasonic thickness measurement
Non-destructive testing Ultrasonic testing Characterisation and verification of ultrasonic thickness measuring equipment
Book: Thickness Measurement with Ultrasound (German Society for Welding)
1998, DVS-Verlag, Duesseldorf, Germany. 281 pages
Nevertheless, UT calibration and thickness measurements have to be performed with the same probe.