My first grow 4 clones stretched from 8" to between 42" and 52" - 1000W MH 3200K Sunmaster, RDWC. My double topped mother in pro mix grew from 16" to 36". A lot of stretch I attribute to the MH and no 36 hours dark on onset of flowering.
I'm flowering two clones now (second grow) - a 6 cola in RDWC that streched from 12" to 18" after 4 weeks flower, and a single cola clone in 2 gallons of pro mix, just in the second week of flower, that has stretched from 12" to 17" with more to come. 1000W Hortilux HPS 2100K. Much less stretch.
I'm figuring a 50% stretch through flower, once I get better at growing the strain - you might have luck vegging 18"-20" to get 27"-30" plants, leaving some room for extra stretch your first time around.
.
" i am planning on vegging them about 6-10 inches and then beginning the flowering cycle,"
Your plants won't switch to flower until they reach sexual maturity (around 5 weeks for a lot of strains) and if you put two week old, 6" plants in 12/12 they will continue to veg until they are sexually mature.
Plants topped several times will require a longer flower period to get a multi cola plant to the same height as a single cola plant, so if you top a few times your plant will be much shorter at 5 weeks than a single cola un-topped plant.
I've found that multi cola plants stretch less than single colas, I've never read anything on this but my guess is the plant has to spread the nutes and water collected by the root system over several meristems rather than just one. I'm pulling that out of my ass but something along that line of thought makes sense. I double top my plants, then when I transplant I bury any stem showing by planting deeper - this keeps them a bit shorter.
If you clone you can put them into flower as soon as they root, so once you get your stretch figured out you can pick the finish height of your plants.
.