I believe the question was about damage that might have been caused by prior use of 7 X 57 ammo in a combo. It was not about the in-advisability of using 7 X 57 ammo in the firearm in the future.
For future use STICK TO PROPERLY LOADED 7 X 57R. Ammo is rare in Canada but not impossible to find, but wait to get proper ammo before firing.
Would prior use of 7 X 57 Mauser ammo damage a combo? Possibly, especially if fired a lot. Whether there has been damage could only be confirmed by a good gunsmith. About all I know what to do is check for a wiggle at the hinge. But do it with the fore end iron off. Hold the butt under your arm and twist the barrels. If there is any movement seen at the breech, have it checked out, or pass.
I have a Brno 304 and actually seldom fire it. Knowing they are versatile, but uncommon makes me think carefully. I only check the sighting before potential use, and I don't use it for general plinking, etc.
I use Norma 175 or 160 gr bullets with starting loads of 4831. Norma bullets are .283 and hopefully keep pressure down.