The transmission cooler fittings are flare fittings so you need a flare to hose barb fitting. I try to use as much of the metal hard-line as possible and only use rubber from one side of the radiator cooler to the secondary cooler and then rubber back on to the metal line after doing this with it.
Maybe, maybe not. A lot of radiators have NPT (pipe threads) in the radiator itself, then an adapter screwed in for the metal tubing/flare to attach to. The NPT threaded holes make it easy to attach any kind of hose/tubing you want directly to the radiator.
