if armed w/ torpedoes, TBs fly at whatever altitude you've set them to, then drop to 200' during the combat resolution. if armed w/ bombs, they act as level bombers - no dive or glide attack, reduced bomb-load at LowN alt.
if you've got DBs & TBs from the same base (or CV), set them all to the alt you use for DBs, along w/ their escorts. this improves the chance of a 'coordinated strike'.
for CAP, i generally use 13-15k', unless i've got 3 or more fighter sqns available. when that's the case, CV TFs might get 1 group flying CAP at 8k', for better reaction against a LowN/torp attack; while a land base might get 1 group at 18k' if its got good maneuver-rating at that alt.