You don't ever "need" to use the transformations in combat, you only need to change into alternative forms to interact with their quest related content sometimes(you'll be told when this is the case), closest thing to needing to fight as an alternative form is the need to be in carbuncle form to lure a monster in the late game but you can immediately swap to something else(base stats are shared so just choose the form you like most, the high stat requirement skills make the slime form the most OP, but other forms are better till then and possibly more fun)
I was just pointing out that the 10 stat tied abilities are separate from the 2 quest tied abilities, just unlock the two quest tied abilities to unlock their individual h-scenes, they'll either appear as quests in the guild on their own(rare), or you need to talk to people in town to hear about extra quest opportunities. Sometimes people have red exclamation marks over their heads this means they'll typically submitting quests to the guild after you talk to them and sometimes they'll just give you clues on how to find extra bosses/hidden treasures(means either unlocks abilities or gives good equipment).
Note: no one will ever scam you if they're not a merchant from the item store building and trying to sell you something it's probably either quest related or a cheap way of getting good equipment appropriate for the part of the story where the purchase becomes possible. Also there's a night phase to the town instead of feeding the slime(ends the day) exit the house to talk to different people.