Lots of questions in here. Yes you can take clones basically up until harvest and reveg a plant after harvest if you leave bud sites on the plant. Cloning in flower is very easily done and achieves vigorus growth when it does finally start revegging. The longer into flower, normally, the longer it will take the plant to root and return to a vegatative state. Also, the success rate goes down the longer into flower you go.
In answer to the bit about needing 10 cm of a branch to clone? Where in the world did this figure ever show up? Yes you want or prefer a decent size cutting to work with but size isn't as important IMHO as the maturity or firmness of the branch you are using. It is very possible to root a little cutting if the stem is hard enough to withstand the trials and tribulation of the rooting process. The softer the branch (fast new growth which is soft to the touch) the more chance you have of it rotting during cloning.
Can you clone from a clone? Most assuredly. You can clone from clone from a clone for generations to come. Sometimes during multi-generation cloning you can run into a little genetic drift in the clones. Meaning that you will notice little differences in the make up of future generations, this is not a hard fact, some strains go years and years with no difference in appearance and some show differences right away. I have one strain now that I am in the 12th generation of a clone from a clone, it is growing exactly as the mother did, another strain started getting smaller buds after, I think the 8th generations and I discontinued it after the second smaller harvest. This could have been me or the plant, hard to tell by a layman.
The only way to become proficient with cloning is to attempt cloning in various stages of growth, making notes of what worked the best for you. Your success will increase and your selection process for where and when to take cuttings will become second nature. The only mistake you can make is not trying different methods and different stages of growth in your cuttings.
There is no shame in killing cuttings in the pursuit of knowledge about our precious girls.