Yeah - aren't chinchilids some sort of rodant? In that case, I would say 0. Mammals, including rodants utilize an oxygen intake system call the lungs. The lungs are unfortunately not suited for breathing underwater. Think about when you are in a swimming pool. If you go underwater you quickly find that you have no means to breathe. This would be the same case for the pet you are wishing to add to your aquarium!
Good luck!
Sorry - someone HAD to do it. It was just to good of a set up!
Seriously, though - yeah 55 gallon and you will want a 4 ft long foot print. Fortunately, the 55 gallon with the 4 ft long foot print is pretty much the standard right now so if you are buying new there won't be any trouble getting it right off the shelf. Heck, even crummy places like Wal-Mart sell them (though they do not general stock very many).
Before you devle into them check out the articles pinned in the top of this section. They are all very useful. Also, be sure to research each of your fish carefully. You may have seen center fish housed with other fish peacefully in the past however, these are often the exception and not the rule. For instance, I have 3 Kenyi housed with my cichlids community - most people WOULD NOT reccommend this. In fact, just starting out, I wouldn't. It took some time just to find the right balance in my tank and there is still no telling as I may have problems in the future (fish can be funny that way). Sure, there is a degree of art to aquarium keeping but, be sure to back it up with the science!