I wouldn't worry a lot about the isolator breaking. If it did the positive FP would push fuel into your gauge. If it did back flow it would be so diluted by the fuel it would still burn off. Considering when I did mine there was less that 1cc of antifreeze in the isolator & tubing. When you fill the cab/gauge side of the isolator consider doing it with the truck running, this will fill the diaphram with fuel , requiring even less antifreeze.
Another tip is to fill it from the cab side with the syringe method. If your guage is higher than the isolator, air will naturally rise up towards the gauge. It took a couple of times, over a week or so to purge 95% of the air out from the guage side. Actually 50% air in the line won't skew the reading much considering it's only pushing 15 psi with 1cc of volume.
Just my . 02. Whichever way you decide to go a FP Guage is worth 1000% more than none, if it saves you a VP44 !!!
Cheers,
Matt