Scenes like the one where Ace took Akainu punch and the one where Ace lowered Luffy's head to avoid 2 Navy soldiers' attacks could have been hints of Ace having FS.
I mean, in the first one, Ace was before Akainu, Akainu then rushed towards Luffy (so at that moment, Ace was logically supposed to be behind Akainu) and yet, Ace succeeded to place himself before Luffy to protect him. So either Ace is extremely fast or he was able to see this event before that happened to protect Luffy.
And we can say the same about Ace having CoA.
We never saw Ace using Haki but we can think he used CoA during his fight vs Blackbeard (Oda had not thought how to represent CoA at this time but in all logic, Ace being a commander and spent at least 2 years in the New World was enough to make us think Ace had CoA).
Regarding his huge Fire Dragon attack, well, it's not as if we saw everything from Ace. The dude appeared in what, 30-40 chapters !!?