Costus and Chastetree

I have a few Costus plants started from seed obtained from you two years ago. The largest is now close to four feet tall and the others close behind.
They are in pots, have not flowered, and have numerous leggy twigs growing from the main, twisted stem
How should these be pruned? Is there anything I can do to encourage flowering?

Costus speciosus can get up to 10 feet tall (3 meters), so yours are still babies! The plant is a member of the ginger family and will therefore make new shoots from a partially underground rhizome. Many gingers have twisted stems, so this is not a disease.The newest shoot should get taller every growth cycle and once it has reached mature size it will have flowers coming out of the newest shoot end. These flowers will be subtended by a bract covered stem. The bracts have a terminal spine and should be flushed with red, while the flowers will be white with a yellow center. Most ginger flowers are pleasantly fragrant. To encourage flowering, the plants have to be grown at their optimum conditions, which is at tropical temperatures and humidity in rich moist soil and full sun. Fertilize with low nitrogen(first number), but high phosphate fertilizer(middle number). It might be best to summer the plants out of doors in the summer if you do not have a greenhouse.
You can prune away whatever you do not like on the older canes, but for maximum strength and earliest flowering, I would do as little as possible. Anything you cut away could be used to make more plants!!!, because to make more plants, the stems can be cut into 3 cm(one inch) sections and rooted in moist sand with milled moss or peat moss added, or the rhizomes (the thickened gnarled stem growing at or just below the surface) can be cut into sections.

I also have numerous (potted) chaste trees which are coming into bud Should they be pruned?

Not if you want to see flowers. After flowering you could do some light pruning.