No problem chopping schefflera, even right after transplant or other root work provided you can maintain good growing conditions (temperature). Looks like some trunks have been chopped previously so you can probably see the type of response to a chop.
Schefflera will bud from any node so it doesn't matter how low you chop.
Like most other plants, it will produce most new buds close to the top of any trunk after a chop so chop close to where you want the new shoots. In other words, cut way lower than your anticipated tree height to allow for it to grow up to the completed height.