Companion planting is a huge myth. Planting a 'trap crop' is different. That means planting a section in your garden of mustards and young brassicas to attract insects such as flea beetles. That really works. Onions do just fine with your broccoli/cauliflower.
The best thing you should do is row cloth. This protects your young brassicas from the flies/moths that lay eggs in the roots. When the larvae hatch that is what they eat; the roots. Get some cheapo row cloth. This protects temperature change as well as not allowing adult flies/moths from laying eggs. This time period is only a few weeks long. Then you can remove the row cloth and not worry about your almost mature gorgeous head of cauliflower or broccoli or cabbage dying because the roots have been eaten. Row cloth is worth its weight in gold to gardeners..
Secure the row cloth with soil so the fly/moth is unable to crawl beneath and lay eggs. Water and light and air are allowed through. I would water well before installing the row cloth (cheap stuff, too).
Tomatoes and peppers will benefit, especially with temperature change. Companion planting? Heck all my plants are cuddled up and love it...BUT. Aeration is a factor, pruning and fans are always necessary for air flow to inhibit fungus. Never a marigold did a thing except attract pollinators. I always have chunks of flowering plants to attract pollinators (anything that flies really).
What are your beds like? Raised I hope without sides and structure? What kind of soil do you have? All soils are super. The only thing that makes any soil better is the addition of decomposed organic matter.
Next year you should not plant any plants of Solanacea (tomatoes, peppers, eggplant) in the same soil. Brassicas the same! At least switch ends of the bed...I extend my garden by using pots; always with potting soil in any pot. There are so many myths out there that are continually being supported and propagated. Please, just come to this site and ask questions. We can help you through this maze of disinformation and help you to be successful.