I mean, there are some that are obviously questionable -- I think a lot of the gen 1 dragon-like mons not being dragons comes down to a design philosophy of making dragons as rare as possible (plus, giving that away as a starter re: Charizard would be
pretty busted), I think we should just take a moment to talk about how a lot of Normal/Flying birds should be pure Flying, and a large amount of the dual-type poison 'mons in Gen 1... did not need to be poison-type. At least Gastly makes sense since it's
a fart a poison-gas ghost. Also I think a lot of problems with a lot of typing things comes down to, "there's only two type slots" -- which is probably for the better, really -- and you end up with the game devs picking the two that hopefully make the most sense (and then sometimes they really, really don't). Example: Celesteela. It has a lot of bamboo-themed elements (due to it being themed heavily around the Tale of the Bamboo Cutter, a.k.a. the Tale of Princess Kaguya (cf. Celesteela's Japanese name, Tek
kaguya)) in addition to being a jet or a spaceship or something, so grass, flying and steel are feasible options, and they chose flying/steel (maybe because grass/steel was already represented among the UBs and it would basically necessitate the addition of Levitate, while all "true" UBs have Beast Boost). Incidentally, Celesteela, Blacephalon and Xurkitree (the latter of which is the only single-type UB) are the only UBs that don't have a double weakness, to match the fairly min-maxed stat lines of the entire series.
So in short, I propose those three get a type combination that leaves them with a single double-weakness. Celesteela as Grass/Flying is easy, but I'm not so sure about the others.
I have a feeling that Flygon's name was a localization error and Flygon was meant to be Salamence's English name. I mean, thematically, it would make sense to go Bagon -> Shelgon -> Flygon since all three end in -gon and Salamence's concept is that it finally learned to fly (and maybe they actually got switched somehow during localization and Flygon was meant to be called Salamence, but I can't really match that name to something in Flygon's concept).
I find your proposition to be extremely unlikely considering
Flygon is also Flygon's Japanese name. If you're right, though, that's one hell of a localization error!
Solgaleo - It's based on the alchemical lion, if I recall correctly, which eats the Sun.
Also: Psychic generally represents cosmic power and generalized "space stuff" (see Lunatone and Solrock, Elgyem and Beheeyem, the move Cosmic Power, etc.), and also magic (when Fairy isn't doing it, anyway). The "magic" type + Steel is 100% fitting for an alchemy-themed 'mon, so Psychic/Steel works, even if it's a little boring considering we've had multiple Psychic/Steels before.
But I'll give the take-haver points for not suggesting Steel/Fire like basically everyone does.