As you have probably discovered, there is a great deal of information available on how and when to prune fruit trees and it is essential to do this correctly, particularly while the trees are young to prevent structural problems with your trees in later years.

Fruit trees are traditionally maintained by pruning when dormant and for stone fruits such as apricot and plum and pome fruit such as quince, this should be done in late winter before bud swell (see http://extension.psu.edu/gardening/fp... for information specific to your area).

Elderberry should not be pruned in the first two years, you do not say exactly how old your trees are but refrain from pruning the elderberry until the third year when you will need to remove the oldest canes which will begin losing their vigor and any dead and diseased canes.

The practice of training young trees may actually lead to less pruning in the future. Training is the directing the trees growth into a desired form. Different training systems are used for different types of tree. For example; apricot is usually trained to a open center system to encourage airflow in the canopy; plum, including prune, is usually trained to a modified leader system (where there is no dominant trunk, or 'leader') and quince usually follows the apple system which is primarily a central leader system (with a single main trunk). Elderberry is slightly different and can either be grown as a multi-stemmed shrub or hedgerow or be trained into a small tree.

Let me know if you would like further details about the individual training methods.