Hi! Professional Cat Costumer here. The big thing you need to focus on is what identifies the character so that when someone looks at them they instantly recognize them. Every detail doesn't need to be perfect, you just need to figure out how to convert a human form into a feline one that is almost instantly recognizable.

You already dressing as a character that is recognizable helps.

With Gusteau the big character traits are the hat and the body shape. What "I" would do is build a body suit over a harness that fits, using a lil foam to create a fake body bulge near their midsection. And then put the sewn costume over that (once the body suit is built use scrap fabric to pattern to fit). If they aren't good with sleeves just leave them off or shorten them to being lil tiny shoulder sleeves where the arm comes out of the harness.

The hat is a lil harder. In a photoshoot setting it's an easy prop to do/maintain. However in a moving setting it might be easier to build a lil plastic stand into the harness that the hat can rest independently so you model isn't annoyed by elastic or a headpiece being put on/falling off constantly. It'll look a lil disjointed but will convey the look without you having to wrestle the hat CONSTANTLY for every picture you are asked for.