I had my BlackBeard ship in my aquarium for a while. My aquarium consists of a betta AND a goldfish, and a crab. (betta's aren't suppossed to be in with other fish because of its beligerent nature, let alone with a goldfish, as a betta is tropical and needs diff water treatment, but I didn't care, hehe). The two got along fine.
And the ship had lots of nooks and crannies for them to explore (which they did). I change my aquarium setup every now and then, so it's no longer in there, but it worked fine.
If you don't wanna put an entire ship in there, they do also make two shipwreck sets, one being the bow of a 3050 and the other being the stern. THe bow is open and has no openings to explore, but it looks cool. The stern is broken (on purpose) and does have windows and openings for fish to explore and hide in.
As for guinea pigs, depending on how big they are, I have actually seen footage on youtube (I can't find it now) of a 3286 ship being used as a house for very small guinea pigs. (it was placed in a tub). The critters would climb all around it and even go into the captain's cabin and come back out. I would never sacrifice a ship for a rodent though. It looked like the bildge rats had grown oversized and taken over the ship.
But I doubt any playmobil would be suitable for guinea pigs as they will eventually grow WAAY too big for it, even for a castle IMO.