76 pixels = safe, 77 pixels = fatal, it'd seem (at (Neo)Lemmix resolution).
Another detail that should be implemented on those levels is that one-pixel diagonal gaps (ie: when corners are touching) can't be slipped through in Cheapo. "Cheapo Mode" also reflects this in NeoLemmix. This is absolutely crucial to some Cheapo levels; in Copycat Lemmings alone there are some that are rendered impossible and others that are rendered trivial by not implementing this difference.
The other major change of Cheapo mode relates to teleporters, which SuperLemmini doesn't support.
Currently, any converted Cheapo level will most likely rely on a VGASPEC file, which would then need to be manually converted to a PNG for SuperLemmini to use (or did you add VGASPEC support?). However, when I finally get around to updating CheapoCon (or replacing it with a more user-friendly tool) I'll make sure to add an option to export to a more SuperLemmini-friendly format, especially now that Cheapo graphic sets can be fairly easily converted to SuperLemmini with the graphic set tool.