Mine eat all kinds of veg. Cucumber is always a fave. You know those bags of mixed leaved you can get at the supermarket? They're a great idea because it means they always have something different.
You dont have to vary their diet but they do enjoy it. PLus it's interesting to see what they favour.
Cat biscuits are popular with my 2. Soak them in water for a couple of hours to soften them and then just drop them in. I stick them in a shallow dish of water to keep them moist.

They dont like citrus fruit or onion, no. I tend to stay away form the more acidic fruit and veg.