@renrose - Thank you for pointing out the pronunciation of [name_u]Taliesin[/name_u]! I had heard someone else pronounce it years ago (and I just assumed it was correct), and they said it like [name_u]Allison[/name_u] with a “T” in front. So like Tallison/Talison. I don’t love the sound of [name_u]Tal[/name_u]-yes-in anywhere near as much! I had been hesitant about [name_m]Toulouse[/name_m], but I sort of like the idea of her having stronger [name_m]French[/name_m] ancestry. I seem to be drawn to “T” surnames for her, so I may go see if I can find a [name_m]French[/name_m] “T” surname I like better than [name_m]Toulouse[/name_m]. I agree that [name_f]Grace[/name_f] [name_m]Toulouse[/name_m] doesn’t sound the best.
@amberdaydream - she’s definitely not just English–I just liked the idea of an English sounding surname, although [name_f]Capella[/name_f] and [name_m]Toulouse[/name_m] definitely aren’t English. 
I had thought of [name_m]Thornton[/name_m] (I sort of liked the idea of borrowing a name from some of my favorite classic literature!), but I can’t seem to get [name_f]Hannah[/name_f] [name_m]Thornton[/name_m] out of my head, even when I’m thinking [name_f]Grace[/name_f] [name_m]Thornton[/name_m] as one unit, one name, trying to attach it to [name_f]Grace[/name_f]'s character. I had considered several others–Townes/Townsley/Townsend (which I think I’m still mulling over), [name_m]Thatcher[/name_m] (I really like [name_f]Grace[/name_f] [name_m]Thatcher[/name_m]; I’m just not sure it’s really her?), Truesdale, [name_m]Bainbridge[/name_m], Trowbridge, Talbot, etc.–but these seemed to suit her the best.
ETA: So I did a little name-sleuthing, and I came across Tourigny, Theroux, and Talbert–do you think any of those work with Grace (and Samuel)? I still really like Teasdale, Townsend, and Capella, but I’m thinking Tourigny and Talbert especially could be serious contenders. If you just saw them on a page, how would you say them? (And if you have a background in French, how do you think the average English-speaker would say them?)