Hehe, well, I certainly don't know! It's too late in the season now to really do that as it has gotten into the nineties this past week. I will cut them back severely in the winter, if I can without killing them and hopefully will have an answer by then. We had our septic tank drained this past week and I had a huge back hoe here digging it up and, Yipee! digging holes, trenches and moving big plants for me. So the carissa holly hedge is on the back burner while I get many new beds finished before the extreme heat.

Will re-sprout from any level of pruning, you could even cut them down to near the ground.

The rule of thumb is that you can cut about 1/3 of the leaf area off at one time. In actual practice some people cut them back to a stump. It the plant is healthy, it will usually come back in either case. If it is in too much shade, it will come back a little spindly anyway. They need some sun to keep their shape and bloom. Prune immediately after the blooms drop to give it the most time to grow and form next years flower buds.

You have essentially two ways to maintain this plant arrangement: as a single hedge, where all the shrubs grow together into a single mass ... or as single shrubs, where each is separate from the other. Regardless of which design you choose, the consistent aspect of shrub trimming is that the base is trimmed wider than the top, so that sunlight reaches the lower foliage and it continues to live. (Where the back faces the house, it will be cut straight and not matter as it will not be seen.) If the top is wider than the base -- as is seen in the shrub at the left side of your house -- then lower foliage will eventually begin to die off and the shrub will assume the form of a tree, as if it were on little stilts. (See this beginning in the shrub, second from right, in your original picture.) In an individual shrub, trimming in a ball shape would be incorrect because it undercuts the base, shading it by the top. A dome shape would be correct because the base is wider (or no less wide) than any portion above it. That said, most landscape designers do not use a series of dome shapes in a line anywhere near as much as they would use a hedge, which is simpler to look at and simpler to maintain. The series of balls is usually the result of a homeowner thinking it is cute and maintaining what would have otherwise been a hedge, in that arrangement. After deciding between a single hedge or 4 individual shrubs, one would next decide on how this is best maintained: through regular, tight clippings designating the outer limits of the foliage ... or through annual coppicing, where all trunks of the shrub are cut to the ground. They quickly regrow with the final result looking simultaneously thick and bushy, and loose and natural. The problem for you using coppicing here is that all stems (trunks) grow out in a relatively uniform length. If they ended up nearly reaching the window bottom, they would also be nearly reaching the door.