I don't think there's any rule about this. It's just a matter of what looks good to your eye. If you'd like to post a picture of the pendant you need a chain for, we can help you with suggestions.
I like my necklaces/pendants to be pretty short, no longer than the collar bone. So I always go with a 14" chain. I bend over a lot to pick up toys and stuff for my kids and just can not stand things clunking me in the face or mouth, LOL
I'd do either one of two things. My open heart is on a round snake chain, but you can also use a wheat chain for that. It would look very nice. If I have some time tomorrow, I'll post pics for you. I'd just stay away from something square like a box chain for this pendant because it's romantic and feminine and has a curve to the bottom of the heart.
I took a few quicky pictures for you stci, just to give you an idea. Your diamond heart is a lot larger than either one of mine, so you can definitely rock a 2mm chain and maybe even a 2.5mm
I did a pair of box chains on my channel set heart. The channel setting lent itself to structured straightness of a box chain. One chain is on the heart and the other hangs below it.
You mentioned an Omega chain, and I thought I'd post one because most PSers have never seen one. I think they're more from our generation. LOL! This one is too large for your pendant, but of course, they come in all widths.
Last but not least, one of my favorites. Practically everything looks nice on this. It's a 14K WG tubular mesh necklace. I hope this helped to give you an idea of choices.