I was able to get things working by using ZWO CCD for both main and guiding cameras after trying all combinations as well.
I have also seen the issue where the optical train drop-down is grayed out - I believe this is a symptom of something not happy in Ekos with your equipment. The only way I have been able to see the Optical Train dialog is to click on the pencil icon next to the drop-down selector - and this only works once all equipment is successfully connected (dropdown is no longer grayed out). Even then it is not obvious that you can change the values in the table that is shown. You have to first double-click on one of the values (for example on the main camera - and it then changes to a drop-down allowing you to edit which camera is assigned. It is not straightforward at all.
I saw lots of problems when I tried using my ASI 120MM connected via USB 3 - once I changed to a USB 2 connector it started working. Yestarday I was trying to get a ASI 174MM working and it was very flaky with USB 3 - it would connect by not successfully take an image. It worked flawlessly with USB 2. I believe at the end of the night I was also able to finally get it to work with USB 3 using a different cable - but I need to spend some more time verifying it working. I have ordered a couple of different USB 3 cables on amazon.