The problem with having multiple Pyramid Butterflies is kind of like the problems people can encounter when keeping multiple Tangs of the same species: We do not know how exactly their social structure works and whether them grouping together is just a temporary arrangement for safety in the wild. We don’t know how much they actually “want” to be together and furthermore, if Pyramid Butterflies are like any other butterflies their sexual allocation is either unclear or gonochoristic (meaning they don’t change sex). Same goes for Tangs, we just aren’t sure whether they change sex or not and many could turn out to be gonochoristic.
With angels, for example, we clearly know that at least the vast majority of them, if not all of them, are protogynous hermaphrodites which means they start off female and a dominant one becomes the male. Pairing and harem dynamics are well known for angels as a family, and one or both are their preferred social units depending on location, availability of conspecifics, and space. This is not the case with fish like Tangs and Butterflies.
I agree with
@i cant think that having an established Purple AND Yellow will present challenges for any butterfly. OrionN keeps a pair of Regal Angels (Wild caught) and a Copperband and he strongly believes that especially in the case of butterflies (his Copperband as an example), they must be established before the presence of active and more boisterous fish. Butterflies, even the hardier ones, may well choose to not eat out of stress. The family as a whole requires someone degree of forethought and special care with regards to tank mates.
So in my opinion, this will be a risk that has to be weighed. If you have the means to do so, you could put the butterfly in an acclimation box and see how the Tangs respond to it, but why stress out the fish that way. You could also temporarily remove the Tangs but that I assume would also take a fair amount of work. I would probably look for something else.
Would you consider some kind of angel?
Both butterflies and angels are beautiful fish. When it comes to colors angels beat butterflies by a small to medium margin, when it comes to body shapes both groups contain many pretty species.
(I see you are not specifically looking for a reef safe butterfly but butterflies in general just don’t stand up too well to any aggressive or even somewhat assertive species, so I would reconsider and maybe look for another way to get rid of Aiptasia, of which I’m not an expert.)