I usually start with something I want to include and how I fit it in depends on whether it is easy/cheap to source and grow, or likely to be difficult. Survival of the fittest is all very well if it's a £5 pack of seeds, but not so good if you're talking about a $50 tree... although I have started a lot of things from seed too! So generally I will try and fit something where I think it will like it, and go for an overall diversity rather than trying to get a complete guild. I rely on my local flora to fill in the niches and chop/drop what gets too much of a good thing (usually grasses and docken for me).
I'd really like more useful groundcover plants and am thinking of propagating more of my perennial kale, but I do have quite a diverse mix of Brassica seed at the moment, so maybe I'll just go wild with that next spring!