2020 4.7 blade (at 405luff) felt a little stiff compared to 2017 one, so i tried it with 370 mast, then it felt too soft, especially at top.

Is it dangerous to use max settings (36cm) for a carbon rdm extension?

Would it be ok if i used 370 bottom and 400 top?
Or is it always recommended to use shorter/softer top?

Similarly, if you have two different brand cc 400s, one 18 other 19 imcs, what difference would it make using softer bottom or top?
Do they all fit each other?

Yes you can mix different tops and bottoms as long as they fit correctly. BUT you need to know what you want and how to get it otherwise it's going to be a lot of trial n error and expense. A max extension will get you a softer mast /sail experience. I run a 480 mast in a sail that needs a 520... works in the light winds but no good as the wind increases and the draft moves back. Good luck

Dear Sea Lotus,
Max. settings is not dangerous for your extension but it is risky for your RDM mast. Always give some tolerance for the mast. It is better to use 400 cm mast for your 4.7 sail.
Bye Melih..

Yes basically the top bends very quickly while the bottom stays straight. I did hear some people use that config with success though, not sure how !

Manuel7 said..
Id always go longer uptop. I have used up to two sizes difference. I tried rigging a shorter top and it looked very flexy. Zero tension down low so huge belly with a flappy top!

36 is fine outside of surf. Mix match so long as junctions fit nicely. You can also make a tip extension to run a smaller mast extension. This is something I've been doing a lot and gives me more rigging options.

I had been using 400 top and 370 bottom for a month now after your suggestions and it feels better for me then full 400, thanks guys.
I haven't tried 400 bottom and 370 top since other option felt good.