How do I mix multipple display modes (i.e. decimal, hex , binary) for individual symbols in the watch windows?
(or is it only a common setting for all symbols????)

It is a common setting for all symbols.
However you can use an attribute pragma to set it specifically for each variable. Not as useful I know, because you have to set it on download, you can't just "oh, I want to see this in binary now.. ok now in hex..."
use attribute pragmas to set display mode per symbol

Well, better than nothing...
"Syntax:
{attribute 'displaymode':=<displaymode>}
The following definitions are possible
Binary format
{attribute 'displaymode':='bin'}
{attribute 'displaymode':='binary'}
Decimal format
attribute 'displaymode':='dec'}
{attribute 'displaymode':='decimal'}
Hexadecimal format
{attribute 'displaymode':='hex'}
attribute 'displaymode':='hexadecimal'}"
</displaymode>
What about display mode format 'ipv4address' ? (yes, a suggestion. Not all of us want to read hex allways...)