Taking species aggression out of the equation, the old rule of thumb (and that's all it is) is 1" of fish for every gallon, so based solely on that you can easily keep the number of anthias you reference in a 900 liter. There are obviously complexities associated with body mass once you start adding tangs and other fish, but your stocking plan seems fine to me--with the possible exception of what type tangs you are thinking about: zebrasomas and similar being on the smaller end of the spectrum, and Acans, etc., being on the larger end. I have a Red Sea P500 and I have three anthias, two clowns, a couple of gobies, two small wrasses and two tangs, and everyone gets along well and the tank does not look crowded and the filtration easily keeps up with the waste (I actually have to dose nitrate and phosphate).