If I can cast "target instant card" from my graveyard can I cast a card that has Flash?
A creature with Flash is not a legal target for an effect that targets an Instant

Phrases like "any time you could cast a Sorcery" or "any time you could cast an Instant" are just shortcuts for timing rules, meaning when you're allowed to cast a card or activate an ability.


  304.5. If text states that a player may do something “any time he or she could cast an instant,” it means only that the player must have priority. (...)


Both the cards you mentioned have you target an Instant card in your graveyard, meaning the targetted card needs to be an Instant itself.

It's possible that cards get types that aren't originally printed on the them (most commonly for animated Lands or Artifacts), but that's not what Flash does.
In general, no Magic card or effect grants the Instant or Sorcery types.