Planting between slow growing brassicas

Does anyone grow anything faster growing between their slow growing brassicas, like Brussels and purple sprouting?

Was thinking maybe some spinach, turnips or if there are any other quicker growing brassicas that would be done before the others get too big?

Or would they take away the goodness in the soil the slower ones need over the long haul?

Cheers

I interplant with fast crops regularly.  For example, I planted out cabbage plants last week and have put tom thumb lettuce in between.  I chucked some pelleted chicken manure on and will again in about 6 weeks, when the lettuce will have been cropped out.

As long as you make sure you feed and water anything you plant intensively like this, I can't see a problem 

As the others have said, I always plant fast growing crops between wide spaced brassicas, usually lettuce, radish etc. as well as marigolds to try to reduce the white fly.
A few chicken pellets replace any nitrogen lost.
Last year, because I use raised beds, I tried parsnips between my rows of peas, which work very well. This year I'm trying parsnips between some of my onions.
The only problem with intercropping is it can make weeding more difficult, some make sure you leave space to get your hoe in or its hand weeding.