Actually I'll disagree and say that I like the idea of Cordelia and Harmony. I see it much more than her and Buffy.
It could have headed that way after Disharmony. The fact that Harmony was even willing to try and control her violent impulses for Cordy shows that maybe underneath all of the shallowness and peer pressure there was something else there between them. I mean honestly Harmony does not get the credit for her restraint in having no soul in that episode. Probably because she betrays them LOL, but still again by vampire standards it was pretty incredible. Could have made an interesting fanfic, seeing where it would go if Harm didn't betray her. It could start out as shallow IE Harm just thinks the cult of vamps are geeks, and sticks with Cordy that way, but in time perhaps Harmony could genuinely fall for Cordy? Only thing is though without a soul or a chip she would just try and vamp Cordy. You'd need to think of a reason for that not to happen. Maybe when Cordy becomes half Demon she realises she couldn't and does a Spike, tries to be good to win her heart. I would just like to say that I would NOT have liked this in the show as it would have undermined the vampire lore, but for a fanfic it might be okay. Maybe Harm could stop Jasmine from happening by being there with Cordy on that night.
Also it's not hard for Cordy to mistake Harmony for a lesbian. "The last time I was happy was with you." Then there is Cordy being unbelievably supportive and even a bit curious about Harmony's new sexuality and still wanting to support her when she is a vampire. Cordelia should know better than anyone how evil vampires are. Spike, Marcus, Angelus, Drusilla, she's dealt with them all up close. Really what was she thinking? I also agree with Wesley, leaving her alone in her home, was a dreadful risk to her neighbours lives.
There being something else there is a better explanation LOL.