There are no hard and fast rules. I think waiting until 7 nodes for the main stem is just to ensure the plant gets somewhat established before you start pruning it.
Notice the overall structure and growth habit of the plant - every spot where a fan leaf grows off of a stem/branch is a node, and in the crotch of that node is a growing tip, a point where a new stem/branch can start to grow from.
If you cut off the end of a branch or stem (aka topping it), all the nodes below where you cut will be encouraged to form new stems/branches, because the outermost growing tip doesn't exist anymore. When the plant starts to flower, all those growing tips will start to form flowers instead of elongating their stems. If the stem is still short, the bud will form very close to the branch it's growing out of, if the stem is longer then the bud will be farther out from the branch.
Just use that logic when deciding where/when to top or remove growing tips/stems along larger branches, to shape the plant to your liking.
You can also bend growing tips downward, which will encourage other growing tips to grow upward faster (on the same branch or other branches below it), without having to remove anything from the plant (aka low-stress training).
If your space is only 2x2x4, you might want to limit how much you top, because it could easily get overcrowded once flowering starts.
Good luck!
