NEED ADVICE, please help!!!3rd grow attempt(pic)

i am 2weeks into my 3rd grow attempt in a stealth grow barrel i constructed....problem is i have only about 18" of vertical space and my plants are like 4" now with the exception of 1 which is like 6-7" and growing faster than all the others combined...at what height should i start 12/12 to get them to be 18" at the finish?also looking 4 any advice to make the best use of my space.....do i HAVE to use nutes at all or can i just use water?...and last but not least id rather not top it cuz last 2 times i tried my plant went hermie,prolly didnt have nuthin to do with that but i feel it did...

when plants go on 12 12 remember the plant is still growing 4 1 half weeks 2 2 weeks
r u on soil if so use soil nutes canna very good very small grow i havent got m head round this site yet but i will try upload some projects ive worked on lol big 1s