<< How can I manipulate the fertilizer so as to make my plants shorter but bushier?
For example, my Basil is leggy, can I use fertilizer with less Nitrogen and reduce this? >>
Hmmm. Sounds to me like you want to keep your "Basil" at max yield while keeping the height under that of your fence. As Fausto said, pruning is your only option at this point. For the future you should look for a hybrid variety specifically designed for "shorter yet bushier".
A certain variety grown in Maui has this quality I think.