Keeping Morish idols is borderline irresponsible but I’ve had decent luck with them and my losses due to whole tank ich wipeouts (and even then the idols were the last to go!!)
if you get a fat healthy one from like Dr.Reef or somewhere the biggest issue IMHO is diet and swimming space….but since you already will have a 300 with tangs. maybe do some research and think about it, the diets are pretty similar..…
I know even suggesting a Idol might hackle feathers around these parts, but again, get a fat one and get it to start eating and IME they are quite hardy and seem to hold their own esp. with nasty tangs…
/….as with anything, the situation and individual fish is variable
EDit Add: gotta be careful with coral selection, if hungry they will go after some…mine leave sps, galaxea, euphilids and most zoas alone….wouldn’t trust them with favia, lords, brains, or many LPS…