thanks 4 answering
I guess the problem is not in gtk, but in the way some programs not made using gtk, renders under gnome.
I found a 'poor method' to not to fix, but to decrease this charasteristic: to set the kde overall fonts couple of points lower, so when such apps are opened on gnome, it fonts shows (just) 2 points more than the rest.
I'm still wonder about 2 things:
1) in which programm language were made apps like slackins, skype and k3b? maybe in sort of 'kde-specific' language? -seems like the problem resides between such language and the "(?)gnome render engine" (or however it is called)
2) why such behaviour doesn't occur under others VMs like fluxbox, where fonts are rendered exactly like in kde? -maybe coz they do not use that 'gnome render engine' ?