First here is an old picture of my tank but it proves that clowns and plants can be kept with each other. I believe the biggest clowns were around 10- 12cm (~4-5 inches) back then.
The most important thing is that they have always some fresh green food available when they feel like eating it. Otherwise they will start eating your plants. Among other food I give my clowns some slices of fresh cucubmer and zucchini and some frozen peas that I have quickly warmed up in warm running water and then I have peeled the skin off. I have also tried some potato and carrot but they seem to be a bit too hard material for the clowns to eat them. Bristlenoses however seemed to be very fond of them.
You can quite safely experiment with other types of vegetables as well as long as you remember to pick the spoils out from the tank so that they don't mess up the water conditions. Another thing is that if you cook the zucchini for couple of minutes you don't need any special tricks to keep them on the bottom. I myself am a bit lazy and just stick the cucumber/zucchini pieces to an old heavy tableknife and I have tied some fishing line to pick the knife up without wetting my hands in the tank.