I have three Little Lambs in a grouping that I cut back pretty severely every year- I have had fun experimenting with different shapes and learning how pruning directs growth.

That said, I am done. I am replacing them with Bobos this fall. In fact I already have the Bobos growing them up in pots- I found them really cheaply and struck while the iron was hot so sourcing in the fall wouldn't be an issue.

One of the Little Lambs is being discarded as it insists on growing deformed leaves. One is being moved to a spot where it can grow it's heart out, and one is undecided- it may end up planted in my woods where it is unlikely to flourish but will make a dandy deer buffet. Not a lot of room for it anywhere else as I already have so many varieties of paniculatas and another two larger LL's down the hill that are going to have to come out sooner or later- they're just too floppy to suit me.

So anyone near me needs three or four mature Little Lambs, bring a six pack and have at it!

I'm with Luis and Cearbhaill - I wouldn't want the chore of trying to keep a 6'-8' plant in a spot that small. If you prune too late, you may remove flower buds. I'd find another spot for these and go looking for something like Bobo, which will stay smaller. It's easier to move them now, when they haven't gotten settled in, though if you just planted them now, you could wait until fall.

As the root ball gets larger, the canes will grow larger each season, even if you cut it back to the ground!! Case in point: I tried to grow my Snow Mountain paniculata into a tree and was on the right track, until deer snapped the trunk clean off in the off-season! The following summer, the replacement canes grew a whopping five feet! I've since pruned those (6 big ones) down to about 18" (this Spring) and the plant is already almost 6' without blooms.

Do what the previous posters said and get yourself plants suitable to your max height. Being the sinister gardener I am, I would let all those plants grow into one another with ease, but that's just my personal preference. A plant with a 6' max height and diameter would be most up your alley.