Error for you, but the perfect match for many else
Crystal maiden is too superficial a hero for Cadance even if it seems SO obvious (And because of it's
"Obviousness", some missed how other heroes fit her more). Crystal is not ice, and with CM's power revolving around ice (Freezing field?). And even if we try the crystal matchup, Rarity would be more fitting when it comes to controlling crystals (Leader of the Crystal Empire =/= ability to bend crystals to her will).
Holy knight on the other hand? Cadance is the representative of love, and spreads it around the land just like what Chen did with his religion. Cadance's power is based on her love, and how it is able to influence those around her (Holy Persuasion). Chen has a more of a leadership role, just like Cadance is the leader of the Crystal Empire. Finally, as I match Shining Armor with Omniknight, a very powerful combo revolves around "Guardian Angel" + "Hand of God / Hoof of Goddess" which matches the pair so much!