Taller and shorter plants

i got 5 plants going in veg (24 hrs/600w light), 3 different strains. 2 of the plants were started late and are 6 inches shorter than the others. My question is, is there a way to slow the growth of the older plants to let the shorter plants catch up. Has anyone had experience with this?

Why not top the taller plants, should let the shorter guys catch up. You're still in veg so you can really abuse them without any worries if you have never topped before.

put something under the shorter ones to even out the height

I though of raising up the shorter ones, but it wont work for what I want to do. The taller one is about 12 inches and I want to start flowering before it gets any taller (limited space). The other one is only 6 inches but I want to wait till its 12 inches before flowering.

If anyone has experience topping this size plants, can you tell me how to?