I normally find a population that varies between from about 120-60 bots, works best in balancing speed with evolution. Still kind of a trick to keeping them all from an extinction event. Maybe something with more bots would be better, I just get annoyed seeing the sim crawl along.
I have to second Nums on the need to experiment with costs. In general the bots seem to care about direct actions the most, presumably changing the costs of storing and shooting would affect them the most.
Making everything free might also prove as interesting as increasing the costs, since this would allow the bots better chances of surviving.
I kind of like cancerous veggies myself. The toughest of these propigate themselves nicely, without the unrealistic repop.
Cancerous animals require almost a constant food supply or something blocking their "reproducing area"; normally it's only a short lived phenomenon(only in systems where food is freely provided, like our ownselves :wacko: can they survive long periods).