Just so we are on the same page, we are talking about the seastar (starfish) in the upper right hand corner, right? THAT does not even slightly resemble a brittle or a serpent star. We have so many common names that are used for so many very different species that it gets very confusing and more often than not chain stores have the wrong names. I looked in my inverts photo collection and find serpents but no pictures of a brittle. I know I have a video of Pesky (red brittle) but let me look again for a still in KaySoh or SueNami's collections. Well, shoot, I have thorneys and serpents but no brittles but here is a
Google photo link to what a brittle looks like. A
serpent star looks similar but without the "hair". Both are octo safe with the possible exception of the green brittle (and I am surprised I don't have a photo of Mr. Green Jeans, he is my oldest critter and now has to be kept in his own tank). Here are some photos of
Bahama starfish (note the thickness of the body and how the arms join the central disk) and here is an
Atlantic Common Star (also octo safe but may star in the tank). Perhaps you could trade the star for one that we know is octo safe? Another option would be a thorny (my favorite for an octo tank). Some are bright orange and they are diurnal (stay out during the day) where the serpents and brittles are mostl nocturnal (except at feeding time).
Shame to give up the condy but it would be much worse to loose the octo to it either from sting or directly though capture (not sure a condy could/would do either but I just would not risk it, the aculeatus are not as robust as any of the others and give the impression of being almost dainty in comparison). We will just have to infect you with MTS (Multi-Tank-Syndrome) once you get rolling so you too can have an anemone tank on the side
Mine is a 4' tall x 16" dia hex that is a pill to clean but one of my favorites.