I have had a few copperbands in with clams. It was likely looking for a free meal, just like the bristle worms.
Tahitians can do OK with some pretty big caveats. By OK, I mean 50-66% success IMO/IME. I got 2 out of 3 Dr Mac clams to live more than a year before I moved. I am 2 for 2 since about Christmastime on the ones in my tank. They need super intense light at the wholesaler (or sunlight) and the LFS - they don't get these most of the time. Most need FW dipped and assumed that they have PM disease - a healthy clam will survive this no problem and it is easy to do. They also need some serious MH lighting - I don't want to get into a whole debate here, this is just my opinion and experience alone, but I think that you will struggle with just about any other kind and I won't buy a Max of Croc under LED at a fish store. They also need a LOT of DOC, so I don't skim, carbon or GFO anymore on my SPS tank, I let the clams filter the water - they are mostly in clean coral systems at the LFS or wholesaler. When they start to grow, they can REALLY, REALLY use up the calcium and alk, so I use a Calcium Reactor - 2 part of kalk cannot keep up if they start to take off. Those of you who have been by have seen my tahitians - not the best patterns, but they are alive and still pretty cool. I need about another month and I will feel safer - they both are starting to put on new scutes, so that is a good sign. In another 2 months, I will cut off the rock rubble that they are on and mount them in the tank for good.