I'd agree with thunderhorse about not growing Solanums near poisonous ones, i.e. datura brugmansias nicotianas etc. That said I've grown nicotianas and daturas near tomatoes and chillis with no ill effects. Or maybe I don't remember them? ;) A fence isn't going to stop anything miss-meandor88. The possums... well I wouldn't mind culling a few to be perfectly honest, they eat all my edibles here. You'd want to use a different area of the garden or a different bed altogether. Aconite and mandrake are some of the few VERY toxic ones I'd be somewhat concerned about. Just don't keep them too close to any edibles/medicinals you plan on consuming. But by that I mean not directly beside them, the chances of soil leaching are pretty damn slim unless you're planting in dense clay. (Which in Brisbane is not out of the question!)