Hydrangea bushes can be trimmed back as soon as they have finished flowering. If you live in an area prone to frosts, then wait until they are over before pruning.
In Spring after worst of frost has gone.
Only to contain them within the space available. The only pruning needed otherwise is to cut off the previous years flowers in the early spring.r
Yes.
yes
they change colour with the soil pH level.
yes
I posted a link that has everything you need to be succesfull in you Hydrangeas planting in the related links box below.
No.
In gardening
How much to cut back your hydrangea depends on the variety of hydrangea you have, and where you live. In northern climates "Annabelle" and "pee gee" hydrangeas can be cut back to eight inches, and still produce a three foot-high shrub with flowers during the summer. "Mophead", "Lacecap" and "oakleaf" hydrangeas set their blooms in late August, September or October, so you have to prune them in early August to be sure you aren't cutting off next year's blooms. Prune out up to a third of the stems of an older plant to encourage new growth. Prune those stems to the ground. Otherwise it usually isn't necessary to prune them at all. Just cut off the spent flowers (deadhead) down to the next stem or mature leaf.
Aqua blue.
Hydrangeas
Take out any old or straggly canes completely,and cut back the branches that have flowered this year to just above a pair of leaves or their buds. In the older types, you can do this back to leave 3 leaf nodes, but if it is a newer hybrid be less severe.