I wouldn't put the Purple Sensations in clusters because if you don't cut off the seed heads, clusters will happen all by themselves in three or four years. Purple Sensation self-sows very happily. Gladiator blooms later for me than Purple Sensation, but it was the first year for all of them in my front garden where I could compare. The Allium I REALLY liked from last fall's efforts was Allium christophii. It's just plain cool.

I added Allium atropurpureum and Allium schubertii this Fall to confuse things even further--also some western native Alliums, but they aren't nearly as large.

Contrary to the previous poster's opinion I would plant all your 7 Purple Sensations as a one cluster.
You'll have a good display the very first year vs 1-2 'no impact' alliums here and there.
Yes, they multiply and seed pretty freely, so after the second growing season you'll have enough to divide and move to another areas.
Beware that PS foliage dies and look very ratty well before it start flowering, so you may want to plant them where it will be hided.
In a picture below (mid-May), dying foliage of alliums is hided by still good looking foliage of daffodils that were planted in a same hole, but bloomed much earlier.
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Same spot three weeks before (you may see alliums flower heads between daffs):
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I agree that single Alliums are very boring, with the possible exception of a few extremely large-flowering forms.

EGO45, that is a nice looking cluster. I like the idea of using the daffodills in the same hole too. How about the Gladiator? Does that look good singly?

The package lists height but not head size. From your photo I'm guessing 3-4 inches for the PS. How big is the head for Gladiator? I hope the colors will go well in case bloom time overlaps.

As shrubs_n_bulbs said, single Alliums are boring. You've seen how beautiful they are in a cluster in EGO's photos. I still think even extremely large Alliums such as 'Globemaster' are still boring planted singly. I would plant at least 3 of them together. 5 or 7 are even better. 'Gladiator' is not as big as 'Globemaster', just taller. I would plant it in a cluster.

I have A. cristophii in front, sensation in back planted as "soldiers" in one bed and it looked spectacular this spring. You need LOTS to make this work, I have maybe 75 or more of each.

I don't think single alliums have to be boring (but usually are). When I was a kid my mom had one single christophii in a huge perrenial bed. When it blossomed it became the "star" of the bed and we waited for it's bloom each year. It kind of hovered over the rest of the bed.

Still, a clump is certainly an easier way to get impact.