I wouldn’t worry too much about it, actually. So long as your uncle makes it clear to his machoke that they’re supposed to listen to you because you’re his relative, you should be fine. If they do show any attitude, be firm and speak with an authoritative voice. Don’t let yourself lose control of your temper, especially thanks to the fact that you’re dealing with fighting-types. With all pokémon, if you’re firm and in control without shouting or showing aggression in any way, they’ll instinctually follow your orders.
When it comes to tauros meat, I have to say that I’m … not actually sure. Tauros meat is a delicacy in many regions, so it’s possible that he’s not joking. Otherwise, the only other reason to keep tauros is to breed them for work or training. You’d be able to tell the difference through whether or not they’re branded. If they’re branded, they’re meat tauros because they won’t be trading owners; if they only have a tag fixed to their horns or ears, then they’re work or training tauros.
As for custap berries, actually, no! Although custap berries are very sweet, they’re still not as high in fructose as added sugars. You still shouldn’t eat a lot of custap ice cream, but that’s true with every food. Keep your diet healthy otherwise and stay active, and you’ll be fine!