keegz i hear your frustration over the fact that your original question isnt being answered and that you feel you need to defend those two dotty backs in such a small system, but the guys are just trying to forewarn you that you will most likley have problems in the future and are just trying to help you. I agree with the others that two dotty backs (especially the same kind) in a 350l system will have trouble sooner or later and I'm afraid with dotty backs once they start fighting its normally too late to split them, they simply kill each other fairly fast
take a note of what so many people are saying there is wisdom in the advice, but by all means do what you think is right. It is worth noting with dotty backs that those which are captive bred tend to be far more docile towards each other (and other fish) and perhaps the fact that you got them younge and added them together means that they are tollerating each other and grouping together for protection. But the day will most likely come that this will stop.
regarding your theory on why tangs fight you are partly correct, although riaan is more correct
tangs of similar body shape have a tendancy to be far more territorial towards each other and are likely to fight. the reason for this is predominatley over territory in order for them to protect the feeding grounds they have and fish with similar body shape is most likely to be in more direct competition, and thus they box.
with regards to adding 4 anthias to this tank with the current stocking you have I would advise against it. the tank being 350l is on the minimum side with regards to their swimming space requirements and the fact that you need to feed them small amounts several times a day would mean that your tank might be easily over stocked. Personally i wouldnt reccomend it.
hope that helps