You can chose any Hudson Reed shower head to fit this product if you wish. Smaller or bigger they'll all fit using standard 1/2'' NPT fittings.
On this particular panel the bodys sprays aren't adjustable. However they are staggered upwards through the majority of the panel and you shouldn't need any adjustment if you get the installation height where it's comfortable.
Unfortunately not. We recommend that you remove all your existing shower/faucet features and plumb this straight into the household supply.
This panel has a very low operating pressure. Needing only 15PSI to suitably function we doubt you'll have a problem. From our research the average US street water pressure is substantially higher than this.
Each control handle controls a different function and gives you the opportunity to combine functions or use them individually. Traditionally you could only use one or the other but Hudson Reed shower panels are pretty awesome!