I'm generally not a big fan of the idea any small animals like that being "walk-through." I know it's a game and that guests will automatically not step on them... But I dunno. Guinea pigs, rabbits, and the like are just so naturally shy that having them be walk-through feels fully inappropriate to me. A family of guests spending time with an individual animal in a controlled environment is one thing. But just having to live with all those giants walking around...
Beyond that, guinea pigs just kinda have such a "domestic" feel to me that I'm not sure they'd fit in a zoo game. With rabbits they could implement a specific, larger, more exotic species that would be domesticated (but would still feel "special"). Whereas with guinea pigs... I dunno. It would always just feel like a pet, if that makes sense.
I would actually lean more toward chinchillas for a smaller, pet-like animal. And I think they'd make fun habitat animals, TBH! Also a bit more versatile as they could be housed in a children's zoo, small mammal house in a more "natural" enclosure, a South America or mountain section, etc.