What tangs do you have now and which do you want to add? Ive heard of rescaping to switch up the territory but have never done it. You can also leave lights off along with other things but the species is what is comes down to. Adding a yellow tang to an existing blue hippo wouldnt worry me a bit. but adding an achilles to an existing powder blue would probably last a day or two. Tangs vary sooo much by species and individual fish but most of the time they will only fight for a couple days so feed well at the beginning and also have the lfs feed them before you buy them(to make sure they are eating and healthy).