Is there a way to keep in app always in front of other apps?
You could try to achieve this functionality with an application like "Afloat" as you mentioned already. But it seems to only work in El Capitan if you permanently turn off System Integrity Protection (SIP) which isn't recommended.
However, here is the link to the program: https://github.com/millenomi/afloat
How to turn off SIP: https://github.com/millenomi/afloat/issues/22