How tall and how quick do the grow?
Realistically in normal conditions you can expect a growth of around 60cm (2 feet per year), ultimate height can be upto 5 metres, we recommend an annual trim to restrict height.
When and how often do i need to trim them?
Trim at least once a year preferably in late summer, a second light trim can be done in early spring if required.
Do i need to fed them?
Like all plants they will respond well to some aftercare, assuming you added food or fertiliser to the planting area adding some topdress annually in spring goes a long way to maintaining a healthy plant, its not a necessity, but who doesn't like a bit of tlc.
Ive not pruned them for a few years and they look very untidy can i prune hard?
Yes assuming they are mature plants which it sounds like they are they can be cut back hard.
Can i plant in all soil types, my soil is very alkaline?
Yes they grow in all soil types but Chalky soil should be avoided this can cause the shrub to become chlorotic (yellowing of leaves)