I am adding a cooler in series with the radiator cooler, and will have to cut into the factory hardline setup. I know that regular rubber hose and gear clamps are a no no, so what I am wondering is what kind of flexible line would be safe to run for temp and pressure in trans cooler lines that can join up to compression fittings?

I bought stainless transmission lines from Summit with AN fittings and it one of the best upgrades I’ve e ever done.

-5 Teflon hose and ends. Cut and flare 5/16 tubing to make AN stuff work. Bullet proof

You could get a nozzle fitting and flare the hardline where you need to, then run the rubber hose.

Are you sure you need it ?
I put a mechanical tranny temp gauge in my 1998 1/2 Ran 3/4 ton 2WHD, that trans never got above 140F running empty in the Mojave desert in the summer time
Other guys had that same issue with non Mopar cars and trucks also, ATF should run at a minimum of 160F according to the oil companies tech people

Earls, Russel, Jegs, Summit, all make a compression fitting to AN adapters to go from hard line to AN. Use -6 then use teflon lines for the flex. Buy them premade from Induction Solutions or other nitrous supplier. It will make a nice appearing and safe installation that will last a long time also.

They make a rubber line that is specifically for Transmission/power steering as the temps of the fluid are higher.

Also you'll need to put on a bubble of some sort, partial flare, to keep the line from blowing off at some point .

Decided to buy some -6 PTFE hose and fittings to run the lines from my rad to cooler, and cooler back to the trans (Street car) instead of messing with the factory hard line to compression fittings on the return side. Still using the factory hard line feed to the rad. Converter is a 3500 stall, and yet to be ran, but the way I see it, keeping it cool overall definitely shouldn't hurt, and its just a dinky heat sink style cooler (trying to keep airflow to the rad and a/c condenser) so hopefully it does help it some. I could not find anywhere that carries trans hose I would be comfortable with using- some would list it, but then the temperatures it was rated for were quite low.