Hi!
Guinea pigs are very different to other small rodents, they aren't climbers like other people have said. Think of them as very similar to cows - they are ground roaming grazing animals - not burrowers. So they won't use the ramps or other floors. Lots of owners on here say their Gpigs do not use them at all. This is why they need at least a 120cm cage (extra levels are not counted)
Guinea pigs can grow fairly large and need lots of room to run around in, so if you put a Guinea pig in that cage it would be very unhappy and potentially get ill and I know you won't want that for it.
Guinea pigs are a lot harder to look after then rats and other rodents (I've had rats, gerbils, hamsters and rabbits too) they require hay 24/7 (which is expensive) so their tummy stays healthy and their teeth don't get overgrown - their teeth can grow into their heads and tongues which would hurt a lot, and they will not be able to eat so the would starve. This is quite hard to see so you have to moniter them daily to health check them. My Gpig had overgrown teeth (even though he was fed hay all the time) which cost alot of money at the vets to fix (£200) but it didn't make him better in the end, and so had to be put down. I had to syringe feed him every day for a week, which was upsetting for him and very stressful for me. You always wonder if your doing the right thing for them.
My other gpig once had some sort of wee infection and it seemed to hurt him when he weed. This meant a few visits to the vets, lots of money and lots of stress for him.
If a gpig gets ill and doesn't eat for a few days it can kill them because they can get a condition called 'bloat' and it happens because their guts shut down.
They have to be fed specific things and you have to learn which foods are also dangerous for them too.
Shops sometimes sell poorly guinea pigs to people too. I had to tell a staff member at pets at home once when I noticed one of their guinea pigs was very unwell (it was ruffled, had runny eyes, walking very slowly and was twisted to one side a bit)
Gpigs can have lots of expensive, complicated and sad illnesses, even if they are perfectly cared for. It's a lot of responsibility having guinea pigs and you have to make sure you can afford the vet bills, which will 100% happen at some point.
Rats are really cool animals, could you not get more rats again? Seeing as though you already have a cage :-)
Hope this has helped :-)
