Google does indeed look at Class C addresses, BUT it doesn’t penalise you for having more than one domain on them. It is very common for hosting providers to share Class C, even the same IP address, for multiple sites and Google respects that. However as Villa10 points out, if you share with others you run the risk of being blacklisted if they do, although its commonly thought that this no longer applies at Class C level, only at IP level.
Regards interlinking them: Google can detect it as it can with 3-way recips outside the same block but it does NOT penalise them. AT worst, you will find interlinking has no benefit to Google.
You need to check your canonical URL out though and check the www / non-www redirect appropriately. That is far more likely to damage you.