When I was piecing together parts for my -6AN transmission cooler lines I was doing it for a 4L80E swap using the OEM radiator cooler and a Tru-Cool M7B which used 1/2" NPT fittings, so I'm really no help with the Imperial cooler you have or the factory AUX cooler. Or the fittings on the 4L60E for that matter. I'll try and list out everything I researched that will be useful to you.
The radiator trans cooler fittings are 5/8"-18 inverted flare, I used these Vibrant Performance 16436 5/8"-18 inverted flare to -6AN adapters.
https://www.summitracing.com/parts/vpe-16436
As for hose I used Russell ProFlex. Probably a little bit overkill but it looks really nice and I haven't had any issues. I bought a 50' roll and ended up using a little over half of it I believe.
https://www.summitracing.com/parts/rus-630270
All my other adapters and hose ends were Russell as well. Everything went together nicely and the black finish on all the adapters and ends look really nice.
Russell 613165 90 degree swivel -6AN hose end: (I used a ton of these, five to be exact)
https://www.summitracing.com/parts/rus-613165
Russell 610025 straight -6AN hose end: (only used one)
https://www.summitracing.com/parts/rus-610025
Also I'm not sure if you meant to type this out the other way around, but in your post it looks like you're talking about running the trans fluid through the AUX coolers first and then the radiator cooler. You'd be much better off running through the radiator cooler first and then the AUX coolers, that way the fluid going back to the trans will be as cool as possible.
Hope some of that is helpful.