You will have a good range of sizes which is always handy. I have grown many varieties in clumps and they all grow fine, from flat Stuttgarter bulbs to Long Red Florence.
18 modules per tray sounds plenty big enough to me, but I am tight on space and am using a 77-module tray this year, which is a little deeper at 5cm. Onions do enjoy some extra compost and I saw some amazingly huge plants at West Dean last weekend, which were sown at Christmas. Mine were sown mid Feb and look good for planting by the end of March, as quite small plants.
