Don't worry about the alternating nodes. That'll come when you start the 12/12/ light cycle. All you need to do is decide on how tall you wish to grow them taking into account how much room you have for them to stretch - they will continue to grow in the first few weeks of 12/12 flowering cycle. Some varieties put on more growth in 12/12 than others. Sativa seems to like to put on another 1/3 to their height and indica less. Your plants look to be primarily an indica strain. Sativa has more slender leaves and grows much taller - generally.

http://upload.wikimedia.org/wikipedi...ndica_leaf.jpg

Cannabis sativa - Wikipedia, the free encyclopedia

Cannabis indica - Wikipedia, the free encyclopedia