Can I grow veg in tubs

You can grow most veg in tubs or pots, depending on the depth of soil and how often you water and feed. I'm a bit worried when you say you are planting on a fibre glass flat roof? Are you sure it's strong enough for all the weight of compost and water? Tubs weigh a lot when planted up and wet.
Both dwarf beans and peas do well in pots, but longer lived vegetables like brassicas always struggle. Carrots would be a good option, and easier, baby beet, which are just as sweet as carrots and quicker and easier to grow with their large seed. Also, from mid summer onwards, there are lots of varieties of Chinese greens which can be used as cut and come again crops, such as mizuna greens or pak choi. They give a good crop for the space. I find baby turnips also give a good return for a small space.
The more you reduce the recommended row spacing, the smaller the individual vegetable will tend to be, or the more you will need to feed and water. Generally though the recommended spaces are for 'rows' in gardens and when planted in the square you can use less space.