Are Hibiscus plants/flowers sensetive to Walnut toxicity/jugolone? I have...
Hibiscus are listed in the following as tolerant to juglone:
Here is another list that gives some ideas for other tolerant plants:
Another thing to consider is the amount of sun that the plants would get. Hibiscus need more light than they might receive under a tree unless they are out far enough to not be blocked by the canopy.
Here is some good general information on planting under trees: