Ashley,
Take these pointers as suggestions and come to your own conclusion with your specific tank with other people's experiences in mind. If you are having no issues with your shark and your other fish yet, wait it out. My brother rainbow shark in his 36g tank is 2 years old and barely 4 in and is a low energy shark like yours and doesn't pester his other fish. If the shark does get too big, the LFS always give you more in return for a bigger fish and they can turn around and get more for it as well. Though in general the sharks are semi aggressive and do grow to a larger size, this is not specifically your fish in your tank.
Water change wise will depend on your tank and equipment. Getting a bigger filter that moves more gph will make water change frequency less often. I have 2 discus, 5 plecos, 4 tetras, 2 guppies, 3 Japanese algae shrimp, 5 Cory cats and a horseface loach in my 60g and only do a 10-15% water change a week. And my filter pushes 400gph an hour.
I would get that API master test kit to monitor your water quality and you can determine how frequent your water changes need to be and at what percentage.
A lot of people know a lot of info about general tank care and fish specifics on her, but you are the only one who knows your tank. I have seen numerous tanks that aren't supposed to work that are healthy and flourish. Don't jump to any conclusions because someone says your doing it wrong, you can determine that on your own
Good luck!!!