Seriously, the commercial tailpiece is not the Mona Lisa, cut that crap off and install the 1/4" female flare, buy the tailpiece to flare adapter as directed, and then you can attach ANY type of adapter at any time ;-)
Also at the same time make sure that you are using the proper length of 3/16" ID thick-wall beverage line. Some setups come from the factory with incorrect line size and length and I have no idea why, but I keep seeing it on half the systems I'm asked to configure.