I really would not worry too much about the possibility of dogs or cats coming to harm through poisonous plants. They are carnivores and not in the habit of eating plants. Both cats and dogs enjoy chewing grass, and there have been cases of cats which were kept exclusively indoors getting poisoned by chewing certain house plants for want of other greenery, but outdoors this isn't really a problem. I am a professional gardener and an animal-lover, so I would be quite aware of this issue.
Box hedging (Buxus) certainly is no problem; it has wiry, hard branches and leaves, and cats and dogs don't eat it. Also, a lot of 'toxic' plants are not lethally poisonous and would make an animal feel sick and maybe vomit, but not do any greater damage. Things like using slug-pellets are a greater danger, but even there I have only once or twice heard of a case of suspected (not proven) pet poisoning. A lot of poisonous plants also taste bad (the nightshade family being an example), so pets won't touch them anyway.
You could also plant things cats actively go for, such as festuca glauca (blue sheep's fescue, a low grass), and nepeta (catmint) - they'll be too busy chewing and rolling in this to touch anything else. I'm not aware of anything dogs particularly love - they really only ever chew grass.
As for advice on what to plant - the main thing is to get plants suitable for the aspect (sun or shade), and the soil. For size, don't trust the labels; in ideal conditions, plants usually get a lot taller and wider than what is stated. Be wary of fast-growing plants; they are great for the first year or two, and a plague thereafter, if your garden is smallish...