Heavy/Flashing Blade A Slot Suggestions

I don't think you'd ever prefer their A-slots versions over their Seals (and sadly, Flashing Blade is only available as their former). A-slots have much better and more skills to mix in with, like Fury & L&D. You can always shift around Seals, unless you mean you want both on the same team.

Flashing Blade's probably even worse as an A-slot. It seems easier to reach over 50 Atk than it is 40+ without Seals. Hell, even Mia (in my case, my -Atk one) is better off with Heavy Blade Seals, since it allows for L&D.

One option exclusive to Caeda and Clair is the flashing blade on their prf effect refine. You can galeforce using that, but maybe only once because they're not likely to survive more than one counterattack (unless fortify fliers AND exploiting colour advantage against a dragon, then I think they're good for getting hit twice, but you're probably not going to find two dragons of the same colour unless one is Ninian) so they need to go desperation once they get hit. The other issues with galeforce on these two is that they tend to need their specials for damage, so the strategy will work against armour and cavalry teams, otherwise you need to stack multiple flier buffs to push their attack into the mid 50s.

Restrictions on flashing and heavy blade is what make them "inferior" to the HB SS, since HB a skill is on 1 hero (I believe) Young Ike, while flashing blade has 1 source as well in Mia
HB can be put on all heroes except staff, while FB cant be put on fliers or Calvary or staff users
Galeforce is niche to units that have other ways to gain charge, or with movement that can be abused (like Brave Roy, Legendary Ephraim) or 2 attacks with QP
As far as your specific request plenty of units can do it, depends on your fodder and desire to run galeforce, I run my seal on dual SS Siegbert with desperation since he'll get moonbow every fight (+ark-Res)

I think Galeforce is primarily used for the first turn galeforce popularized by B!oy. Since flashing blade is infantry&armor&caeda&clair only, it is difficult to get the "first turn galeforce" in, and difficult to follow up except with maybe caeda and clair. As they have flashing blade built into their weapons they can run any A skill of choice, but cannot run the safer option of firesweep weapons, so may die due to their low physical bulk.

Therefore it might be better to look at high attack and speed melee cavalry/flying. Due to lower BST of cavalry/flying, a legendary weapon (like B!oy's) is helpful. iirc Siegbert with Heavy blade A skill can function similarly to a "skilled Roy" (with Swift Sparrow and Heavy blade swapped between their A skills and weapons).

With -Spd on Soleil, I think it might be better to not run Galeforce due to the loss of significant doubles due to 32 base speed. If you insist a Spd refined Slaying edge can raise it up to 35, which is still a bit low for a hyper-offensive unit.

Lastly I think there were a few gimmicks such as Heavy Blade Cherche and Effie to fire off low CD specials every second attack from their brave weapons. I've never seen them in use so I can't attest to their viability.