emma, welcome to AC! :welcome:
unfortunately what you are experiencing happens with many goldfishkeepers. don't necessarily hold it against your local shop - most LFS employees simply don't understand the real-world requirements for adult goldfish. a few may outright lie to you to make a sale, even if they are nice people. as a general rule, LFS employees are not to be trusted.
remember that they are not experts in fishkeeping, they only sell the equipment. much like you would not expect sporting goods store employees to be able to give you the best information on how to train for a marathon. most LFS workers do not have much experience at all in keeping fish. unless you know your LFS employees have past experience working with public aquariums, research facilities or a long history with private aquarists, they won't be able to give you accurate information.
black moors can grow to much larger sizes than you might realize. your fish will easily reach 8-10 inches long if you provide them enough room and quality food. but goldfish kept in tanks that are too small will become stunted, which causes illness, organ failure and premature death. also, since moors have telescope eyes, they have relatively poor vision and cannot evade omnivorous predators like turtles as well as singletailed, normal-eyed goldfish.
100 litres, or approximately 26 gallons, is a very small tank even for just two goldfish. a 40 gallon tank, or 150 litres, would be a much more appropriate home for your moors. as it stands now, you should certainly not add a turtle to the setup. You can read a little more about musk turtles here:
http://www.austinsturtlepage.com/Care/caresheet-stinkpot.htm
This care page, which is very thorough, recommends a 29 gallon tank as a bare minimum setup. Since your goldfish can grow very large, you would be at more than double, nearly three times the recommended capacity for your tank if you added the turtle.
Best of luck to you and your goldies. If you have any additional questions, we're happy to help.