Yes Lilli but depending on how thick your fish is, the vegies may take longer so you could just add the fish later.
Just check out some of the fresh fish recipes on the forum for steaming times to give you some idea. For small salmon fillets of 125g, I only steam them for 8 minutes on varoma temp using 600mls of boiling water in the bowl but one of the recipes from MWOC which uses codfish, the potatoes are put in the basket (4cm dice), zucchini goes in the varoma dish (.5cm slices) and thick codfish fillets go on the tray and it is cooked for 25 minutes with 700mls tap water in the bowl. Everything is kept warm whilst the citrus butter is made in the bowl. Fish cooking takes a bit of practise to get right, you don't want it steamed to death. Each time you do a fish dish make sure you write your times down if they differ from the recipe - it takes the guesswork out of it for next time.