Why not only replace the axle? It's such a simple job that even if you have to have a lbs do it the end sum is likely to be less than a new wheel. Besides, as already suggested, it'd provide a nice opportunity to have the dropouts checked.

And no, parts are not routinely mismatched. But 7-speed freehubs seems to be more common than 6-speeds, so someome probably slapped on what they had after a previous failure to get the bike back on the road as fast as possible again. If you have friction shifters it's a non-issue. If you have indexed it might not run cleanly.