I noticed the ostovite's "bone chariot" doesn't list a minimum size for the creature's skeleton it's using. Does that mean an ostovite (who are tiny) can use, say, a fine size mouse skeleton as a chariot? Or is the whole thing just an oversight on the designer's part?
There aren't any specific rules against it. You are certainly right that it doesn't make sense that an ostovite could fit inside the skeleton of a creature that was smaller than it, and it wouldn't be a very good choice anyway, since most things that small are weaker then a larger skeleton.
I don't know that I would call it an 'oversite' exactly, it is pretty common sense that an osovite wouldn't use such a small skeleton and I wouldn't expect a designer to detail out such a rule. Certainly as a GM I wouldn't ever want to have an ostovite use such a thing. If a player someone had control over an ostovite, and wanted to use a skeleton that was smaller than the ostovite for a chariot (something I would never expect to encounter) I would tell them no, that doesn't make an sense.