Clonex works perfectly just remember take a little out use from that that you took from the container after you are done dispose of anything left over so you don't transfer bacteria back and forth to your gel just cut off the branch dip it into the gel for just a few seconds and then place it into a pellet medium soil or a hydro and you should be good to go
As for cutting the branches down on the bottom this is one of the best places to take them from due to the are struggling of lack of light and going to die off in the end anyways another way to do this is when you start doing your topping you can take the top and clone it as well this will give you a clone and allow your plants to split another thing that I like to do is take my clones when I am lollipopping a plant because it is just going to be thrown in the trash throwing it in a cup of water will root a cutting if you are going to be putting the Clones outside it is best to leave them in water for 24 hours before placing them in any kind of soil or medium to root I don't understand quite yet why this works but seems to speed the process up
I can take a clone as small as to inches and I can take a clone as big as almost a branch 12 to 18 inches is the biggest one I think I have ever done but these are more advanced clones when you get this big