The height of the cylinder makes no difference, it is the height of the feed tank which gives you the pressure you need. You also cannot just fit a pump to any system you need enough stored water in the storage tanks, normally when a power shower is fitted the tanks are replaced by a bigger tank or a second tank is married to the existing tank to double the amount of stored water and so preventing the feed tanks running dry when the pump is activated.
Call out your local plumber you should get a free survey and he will tell you exactly what you need.