I know... it's an old thread, but I too am working on a plumbing system that uses flexible tubing, but not vinyl.
My plumbing will be on a 20 Litre Pico tank with 1/2" bulkheads.
Into the bulkhead fittings I am threading in a push fitting like the 'John Guest Speedfit'
http://www.johnguest.com/part_spec.asp?s=USNC23_S3
These fittings do have some stainless steel retaining 'teeth' in them, but the seal is formed by an O-ring that excludes all fluid from contacting the metal parts, and they are thus rated for semi-corrosive and/or corrosive liquids, but they cannot be immersed in saltwater because then the water will contact the metal 'teeth' that hold the tubing in place
These fittings can also be disassembled (ie: tubing removed) non destructively (difficult to do with barbs) so I can do away with unions, and no clamps are necessary
The tubing will be food grade 1/2" I.D. polyethylene
if you heat the polyethylene tubing in hot water it becomes more pliable and when it cools it retains some memory of the shape you bend it to so you can tidy up your plumbing. It is cheaper than vinyl (at least in quantity it is). Also vinyl is more prone to staining, or having VOC's or other compounds migrate through, but the poly is less permeable, and less reactive (just try and find a solvent cement for poly, nothing works well particularly on HDPE). One other advantage is that polyethlene has a lower coefficient of friction than the vinyl so the flow characteristics tend to be better, particularly if you can find the 'barrier' tubing.
I am nearly ready to start the plumbing. I just finished purchasing the last of the parts. I will try and post photos when I'm done.
BTW. GSP what did you end up doing for your plumbing?