I just thought I would like to contribute to this thread and post a picture as well to show you first hand what I see every day in my house.
I currently have 3 rabbits who have the run of my living room. I believe rabbits need a large amount of space and do not like to keep them enclosed in a cage or hutch. I also have 7 piggies who live in the room in their cages who also get to come out and run about with the rabbits. As well as them I have 2 Syrian hamsters in the same room and a puppy who goes wherever she likes lol .
One of my male piggies (Elvis) before he was neutered did not like any of the other piggies and wanted to be a rabbit and loved them more than his own kind. He lived with them out and about in the living room very happily as you can see by the picture below.
This is April one of my female rabbits and Elvis who often cuddled up to her and fell asleep.
He absolutely adores her and she is just the loveliest rabbit. Whenever the piggies come out for playtime she basically lines them all up and cleans each and every one of them. They stand enjoying the clean until she moves on to the next one lol.
Once he was neutered and I was able to put him in with the sows he decided he preferred them to live with but still enjoys time with the rabbits.
I would not recommend anyone put a rabbit and a piggy in the same cage to live together because they are very different sizes and have totally different exercise and food needs and of course they do need they own kind as a mate. However I do not agree that different animals cannot live in harmony together as part of a family.
My hamsters come out for floor time and once again run around with the piggies and the rabbits and the dog quite happily.
The puppy loves to play with the hamsters especially our boy Travis who loves to play and chase her and she him.
My animals are all fed what they require for their own needs according to their species. Just like I do not feed my children rabbit food so I do not feed my piggies rabbit food and so forth.
My rabbits are immunised every year and therefore the risk of them passing on any diseases to the piggies or the hamsters, dog or me and the children is very slight and no more than the others passing anything on to us.
If all animals are fed and their health and well-being looked after correctly then it is possible to have a happy and healthy environment where they can live together. However as we are all very much aware on this forum this is not always the case and therefore guidelines are put in place to help people understand what is now a better life for all species of animals.