Andrew,

While both are in the Allium family, onions and shallots are very different and behave very differently. Shallots are normally started from small bulbs while onions can be started from seed, small bulbs (sets) or plants. I know very little about zone 10 but I'd guess that if you're going to be planting onions at this time, I'd recommend going with plant starts since seed is going to take too long.

You might want to start by checking out the information on a site like www.dixondalefarms.com and see if you're already too late for your spring planting.

We have a garlic farm in zone 7 and grow alot of shallots too. We plant in the fall when we plant garlic. I beleive it would be too hot for the shallots if you planted them now. You plant shallots much the same as you would garlic but give them more space 8". They will multiply. For planting shallots choose medium size bulbs that are not deviding for best results. (we always use sets not seed)
We don't grow onions)
The Garlic Lady