Hi yes pet corrector is very good but not on timid dogs. It's good for hightened behaviour like aggression and over excitement.
The dog borstal is pant's in my opinion. Water will have very little effect on an aggressive dog unless you can catch it the second it's attention is focused on the object of it's aggression. The pebbles are a sound distraction much like the clicker and the discs.
Cesar millan uses the tsst sound and it has the same result. His books are very good as is his program( if you can get around the american production).
What the introduction of sound does is make the dog have an aversion reaction to what ever stimuli you have placed before it.
Walking your dogs individually will help you control any situation better, however by walking your dogs in a pack you can effect all your dogs at once and rehabilitate as a pack.
The key is to rebalance the roles within the pack structure. Yes you love your dogs but LOve isn't the number one thing on their list. they need structure, consistance and rules.
Exercise is major but the right type of walk is key to a balanced pack. That'snot happening here.
Im my opinion it's the structure within the house that plays a key role in mastering the outdoors leadership role. An owner can't expect to have a total following pack if inside the "den" the dogs get away with everything.