I picked up a pear ruby that's about the right size and a stunning red color. I'm a bit worried that it's too saturated next to the diamond though. Of course I'm worried that I bought a lead-glass monstrosity too since I paid $35 for it. I didn't bother getting a cert first since I wanted to check out the size and color next to my diamond first. I only bought from this ebay vendor because they do disclose lead-glass filling. Of course that doesn't mean they don't just randomly divvy up their uncerted stones between treated and untreated and hope no one checks. I'll see if I can get good pics later on tonight.
I have a similar colored FCD and I was thinking of making a bypass with a mahenge, but the general consensus was not to do it, but I do like the colors together.
I ended up putting it in a multi-color FCD ring with other FCD's.
Here's that same FCD with some white and other FCD's in my ring. It's the marquise. Just to give you an idea of the combination of mutliple diamonds together as you were considering.