Mordred should work great in a Poseidon/Nike unleashed team (preferably with Cthulhu too). Then you can fill the 5th slot with whatever.

Ryu-oh in a vacuum is really good. But in your case specifically, Ryu-oh loses a lot of her allure.
So her skills are a 20% A-frame atk debuff, 20% A-frame def debuff, and adding another orb to the enemy's overdrive meter.
These are all great to have one of each of in your team. But same frame buffs/debuffs don't stack with themselves (in fact, the sole purpose of the A/B/C/etc. frame/class/type/whatever distinction is indicating whether they stack or not). And I don't think overdrive extensions can go beyond 1, but that only interferes with Mordred right now if you go with a water team.

Poseidon has a 20% A-frame atk debuff. Nike unleashed has a 15% A-frame atk debuff. If you add Ryu-oh to this core, then you'll have a couple of skill slots wasted on duplication. You can make the argument that duplication serves as a backup in case the primary version fails to land, but I don't think that is as valuable as adding a new function or something that would stack on top of what you have.