Yeah Frederick and Cordelia have a good support conversation chain, it's a bit of fun and feels natural. That's a good idea with Frederick, I've never tried it myself, how's his exp gain nowadays? And yeah you're right about Cordelia, she appears for all intents and purposes to be the best unit in the game as she doesn't need babying, and she has fantastic bases, growths, and join time. Personally I prefer her as a Mercenary/Hero to a Pegasus/Falcon Knight but she works really well in either. There is however one unit that joins that is stronger than Cordelia, though it's not immediately apparent (hence my earlier statement about Cordelia appearing to be the best) and that is Panne. If you give Panne your first Second Seal and reclass her directly to Wyvern Rider you've just created a wrecking ball that'll go on a path of destruction through the rest of the game.
Seriously what even is 105% HP, 75% Str, 65% Skl, 70% Spd, 50% Def?