As someone who does occasionally monitor the Final Fantasy tag I do sometimes have to remove/add the Remake tag from Tifa and other characters. I didn't particularly see an issue or need for your original request since you would just have to -final_fantasy_vii_remake but I see it's not that simple. There's a lot of noise present: EC, AC and all assorted Compilation entries. Adding black_skirt & suspenders helps.
Her original/classic outfit is in the minority nowadays, so it makes sense that you'd want to search for it.
She's not the only who has distinct Remake versions. So do Aerith, Barret, Yuffie, Vincent. And even side characters like Rufus. Cloud, Cid, Cait Sith, Red XIII are more or less the same. Tifa changed the most significantly.
As for the name, I would strongly suggest tifa_lockhart_(ff7_og), because "classic outfit" while being okay might drown in the abundance of tags that Tifa already has. By switching up the format (starting the tag with a FF title) it would make the tag jump out more when people are looking at the tags. ff7_og is also less ambiguous than "classic outfit", the latter could still be interpreted as Tifa being in a classic old-fashioned 19th century outfit, or some such. You can apply this same tag to other characters with distinct FFVII OG designs. Other options: tifa_lockhart_(ff7_1997), tifa_lockhart_(1997).
I suppose an umbrella copyright tag (final_fantasy_vii_(1997)] could work but good points were raised.
"I don't think every post in favgroup #48332 should have Final Fantasy VII (1997) when many posts are based on her Dissidia 012 Final Fantasy appearance."
If it makes reference to Dissidia 012 (narrative) events then it should be Dissidia 012 only, in terms of copyright. Since many entries across the Final Fantasy Vii compilation share outfits and events. You can't tag something with all the copyrights it's featured in.
There is no reference to any specific entry? Then tag the costume/character/event to the entry it was first (MAJORLY) featured in. The parenthesis is because a character could've been teased but not majorly featured like Genesis in Dirge of Cerberus: Final Fantasy Vii but he is regarded as a Crisis Core: Final Fantasy Vii character.
If there is a reference to a specific entry? Then tag it with that.
At the end of the day, it's also up to the uploader/tagger's discretion.
"Would every Remake Cait Sith post need Final Fantasy VII (1997) because his Remake design is near enough indistinguishable from his original design?"
Not particularly an issue to me. For characters with indistinguishable designs it's up to the tagger's discretion. Though the follow-up point is a good counter for not using an umbrella copyright tag. With a vast copyright such as final_fantasy_vii perhaps it's best to err on the side of granularity. In this case the more detailed specific tag tifa_lockhart_(ff7_og) wins out.
"This is also a problem for Tifa's Remake design too. People aren't tagging Final Fantasy VII Remake on every base outfit Ever Crisis post."
If Tifa is wearing her base Final Fantasy Vii Remake and it isn't tagged as such, without any specific references/costumes of Final Fantasy Vii: Ever Crisis present, then it's a case of mistagging.
"Her Advent Children outfit is also her base appearance in Dirge and Kingdom Hearts. Tagging Advent Children on non-AC posts like post #5872565 or post #5545848 because a pictured character's outfit first appeared there seems excessive--At least when dedicated costume tags are an alternative."
As Hillside Moose and my post says, the context matters. Reference to entry => tag with entry. No reference => first major appearance. In this case, by default that outfit would get the Final Fantasy Vii: Advent Children copyright. If there's a reference to Dirge Of Cerberus: Final Fantasy Vii , Kingdom Hearts Ii , or maybe even recently Final Fantasy Vii: Ever Crisis , then tag it with that copyright instead of Final Fantasy Vii: Advent Children.
"I had a long-ass debate with someone in topic #30379 over splitting the Remake tag"
Good times.
If there would be a new character tag it would be useful for characters like Tifa, Aerith, Rufus even. Since theirs are most distinct. This artist, for example, likes to draw the OG versions for the final_fantasy_vii cast.