Things that dont go together! eg blackberries and hawthorne! Eucaliptus and grass! Honeysuckle and>?????
Like pebble, I'd need you to give more info: the things you mention are all pretty damned rampant in these parts, and I imagine the only reason they wouldn't take over together is because one monstered the other first... Actually, agreed on grass/eucalyptus, not that I find it very sad when grass can't hack it! My mum has a 'woodlot' of extremely close-planted eucalyptus. No grass, but I've been surprised at the enormous number of native trees coming up, given eucalyptus' reputation for allelopathy.
Singapore daisy and sanity? I planted sweet potato thinking it would out compete the Singapore daisy but the blasted stuff won. Chickens and vege patches. Not if you plan to harvest anything that is. Teenage male humans and fridges. Nothing lasts in the fridge with a teenage boy around.