Is Dragon Scale just for Kingdra?

Evolving, yeah. May as well. I'd personally max it out because I don't believe in only investing in "elite" Pokémon and you can't go wrong with 100%, but I don't think that's necessary for now. Maybe power up a little at a time

At least in gen2, no other mons require dragon scale.

In my opinion, I reason that since evolution items are rare, it's only worth evolving high ivs, despite the level (like yours!!)

Just a heads up, I wouldn't necessarily power it up depending on its moveset. I got water gun / blizzard, which in my opinion is inferior to the other possible movesets. Any water / dragon combo that gets stab is great, (I think a hybrid of typing attacks, e.g. Dragon breath / hydro pump, will do great in PvP by throwing players off).

However, the aforementioned moveset that I acquired doesn't get access to stab on blizzard. If it was db/blitz it would be better as both attacks hit Dragonite hard, but I just don't think with wg/blizzard it's worth powering up and it instead may be worth my while to re-roll. Best of luck on the roll ~~