Hi
Additional, can the suffix (.html) be switched per component with sh404sef?
This is pre-sale. The answer is no. You can see that yourself because you are using sh404SEF at this moment.
Hi is it possible to use the default sef function of Joomla for custom components and use sh404sef for all other components in both modern and legacy way?
sh404SEF does not have "modern" or "legacy" modes. If you are talking about "Using Joomla router" for one or more component, then yes, you can, as you know already because you are doing it.
Whether it "works" for your particular component, I don't know as it 100% depends on your custom component. We can only work on the URLs sh404SEF produce, not the URLs produced by the Joomla router and your component router.php file.
So as with all custom components, you will have to test for yourself, and possibly adjust either your router.php file or write a dedicated sh404SEF plugin to get desired SEF URLs in all cases.
Best regards
Yannick Gaultier
weeblr.com / @weeblr