It sounds like you are using Monoject brand needles? - a green hub is 18g. For a Shih Tzu you could use anything from 18-23g the 23g are about 1/2 the diameter of the 18g so the flow is much slower.
As mentioned I used Terumo
Ultra Thin Wall 21g for a cat, the gauge you use depends on the comfort of your pet when the needle is inserted and how well your pet tolerates the length of time it takes for the fluid to flow.
If you find your dog flinches constantly when the 18g is inserted try a 20g or 21g, the flow rate will be slightly slower, but hanging the LRS bag above your head helps the flow rate.
Switching to Terumo Ultra Thin Wall will also reduce the discomfort of needle insertion because the walls of the needle shaft are much thinner than regular needles. This allows for both for faster flow and less of a pinch to the skin, the beveled edge aides in piercing skin smother and faster. Because the walls are so much thinner on the Terumo UTW the flow rate on a 21g was about the same as a Monoject 18g, but I didn't feel like I was stabbing my cat for each treatment.
He's probably not too interested in the smell of the KD food & that's why he's walking away from it. What food was he on before the kidney issues? Have you tried crushing a bit of milkbone ontop of the kd?