Actually Ultima Weapon first appeared in FF6 as Atma Weapon, so original source would be FF6 not FF7. Knights of the Round is not FF7 exclusive. It appeared in Type-0 as a summon, as well as some lesser known FF iterations. Hades was also in FF5, FF9, and was a boss in an FF11 expansion; so neither is Hades exclusive to FF7.

It just seemed like you were getting at the idea of bosses being ripped solely from FF7, when in fact many of those characters appeared in other FF titles.

Even though I really enjoy FF lore in general, and I get a kick out of characters from some of my favorite FF games being brought to life again in FF14, I can see the need for original characters as well. It can't all be fan service I guess.