That's the one!
Thank you, I'll remember that for next time, have a heart. 
I'm in two minds about it for now.
On the one hand I like it for the same reason as you said, and the idea that Mythal may have been cheating on Eglarlglefarth (I can't remember the name) with Solas is pretty juicy, but on the other I just think... how is my poor girl going to stand a chance against someone who's as old as Solas is and who'll understand him infinitely more than she ever could?
GAH!
Elgar'nan. 
As for your question.. don't think of it like that.
If Solas and Flemythal were involved in the past, imo it doesn't look to be setting up to be some kind of love triangle or competition between Flemythal and Lavellan. A few things to consider in this regard:
1. Flemythal and Solas' hypothetical romantic connection is almost certainly just a thing of the distant past
2. Flemythal may be dead
3. The depth of Solas' feelings for Lavellan are abundantly clear
4. Lavellan is a powerful deity-like being to some people in Thedas in her own right; in addition to this Solas kind of makes a big deal about how she isn't your average mortal in terms of intelligence, conduct, etc. So I don't think there's much measuring up that would be needed to be done.