Not super familiar with either of these scopes, but I believe the Redcat does not need a flattener/reducer. The newt probably will need a coma corrector. That being said, there's a few things you can do.
For the Redcat, if you want to use a DSLR you will need a T-ring for your make of camera, and I believe the back of the Redcat is threaded so you can just attach the T-ring/camera right to it, kind of like a big lens. Not 100% sure what all of the threading is so you'd want to check that and make sure you get the appropriate T-ring. It's likely m42 or m48. It's a similar situation for the ZWO, they usually come with a series of adapters/spacers and would probably just screw onto the back of the Redcat.
The Skywatcher is similar if you're using a coma corrector, it's probably threaded so the T-ring or ZWO adapter will screw right to it. If you're not using a coma corrector, you'll need a nosepiece that screws into the T-ring (or a T-ring with one built in) for the DSLR, and the ZWO cameras come with an adapter that has a nosepiece which you'll just insert into the focuser.
An example- I use an ED80 with it's specific flattener/reducer. The back of the flattener/reducer is m48 threaded. When I used a Canon DSLR with it, I had an m48 threaded T-ring that attached to the camera and screwed right to the back of the flattener/reducer. Now I have an ASI294 and I use the m42/m48 adapter and the spacers that came with the 294 to attach it to the back of the reducer/flattener. (On the ED80, the reducer/flattener itself screws right to the end of the draw tube).
Hope that gives you some insight. It's possible that the manuals for the scopes give more info on how to do it for that specific scope.