You could probably keep them as high as you wanted to, seeing as they can be used as hedges. If left uncontrolled, though, they will grow to a hight of 12+ feet (4+ meters). When you say Hawaiian Umbrella Tree, do you mean the smaller version (Schefflera arboricola) native to Tiwan or the larger version (Scheflera actinophylla) native to Queensland and surrounding area? Because the larger version can obtain highs of 30+ feet (10+ meters). Either way, one still must be careful about where they plant them, about 30 feet (10 meters) away from foundations and pipelines, as their roots are highly aggressive. I am not sure about what companion plants would work around them, but you could always exeriment there.