@iamasif: "....but for now I want to use Def.Tech center channel which is 8-ohm, is there any harm in mix-match 4/8 ohm speaker."
To your concern above, not at all. You may run into some volume issues which I believe your XMC-1 can adjust so that L/R/C are matched in terms of volume output.
The larger issue, I think, would be significant sound quality or sound signature differences across two different brands of speakers and their specific implementation.
I've used different Centers than my L/R speakers in the past, sometimes with good results. Your ears will let you know. : )