What do you have? I'm pretty new to edibles myself, but have always went for the "spiller, thriller and filler" philosophy when doing a container. In other words, plants that spill over and soften the edges, trailing plants combined with something really eye-catching and "special" and plants that fill in and contrast in foliage and bloom color around them.

I think generally, opposites attract. Plant something fast growing with something slow grow; something with deep roots next to something with shallow roots; something that for example needs lots of nitrogen with something that is nitrogen fixing and so for. This way the plants are never fighting for the same nutrients at the same time.

It's far more complex than speed. in some guilds like the 3 sisters (corn, squash, beans), the beans fix nitrogen, the squash makes an enzyme and the corn provides shade and support) or something like that, that's not exactly right but that's along the lines of it.

google "guild plants" and "companion plants"