Transferring a Pokémon from professor Oak isn't a trade so probably not. The transfer doesn't involve a trade machine or changing the Pokémon's trainer. It would be bending the rules a lot to have Boldore evolve that way.
It's still possible that Ash's final Kalos Pokémon could be a ground type or a type that resists electric type moves. In any case, at the moment none of his rivals or the final gym leader have Pokémon that can exploit that weakness, other than Tiereno and his Raichu (whom Pikachu's already proven he can beat). That's not to rule out an electric type user at the league but for the moment the hole in his team is not too much of an issue.
Bringing Goodra back at this point would mean he would have two Dragon types in addition to three flying types. That would give his team three Pokémon weak to electric types and two weak to ice. There would be no Pokémon to balance out the weakness to electric types and only Fletchlinder to balance the ice weakness. Reuniting with Goodra would make Ash's Kalos team the least diverse he's had in any region.
Well in regards to Ash catching a Ground type pokemon, that would mean Ash would have to catch a past gen pokemon, since the only Kalos pokemon that are Ground-types are Diggersby(the evolution of Bunnelby), which is unlikely for Ash to get considering Clemont already has a Bunnelby, and the only other Kalos pokemon that is a Ground-type is the legendary pokemon Zygarde.
Now in regards to having weaknesses to Ice and Electric type attacks if Goodra comes back here is how those weakness work on the Kalos team:
Week to Electric type attacks: Grenina<-->Ash-Greninja, Talonflame, Hawlucha
Resistant to Electric type attacks: Pikachu, Goodra, Noivern
Can learn or already has moves that are super effective against Electric type pokemon: Pikachu, Goodra, Hawlucha, Greninja<-->Ash-Greninja
Weak to Ice type attacks: Noivern, Goodra, Hawlucha, Greninja<-->Ash-Greninja(if the Ice-type move Freeze Dry is used against it.)
Resistant to Ice type attacks: Talonflame, Greninja<-->Ash-Greninja
Can learn moves or already has moves that are Super effective against Ice type pokemon: Pikachu, Hawlucha, Talonflame, Noivern, Greninja<--> Ash-Greninja, Goodra
Weak to Fairy type Attacks: Greninja<-->Ash-Greninja, Hawlucha, Noivern, Goodra
Resistant to Fairy type attacks: Talonflame
Can learn moves or already has moves that are super effective against Fairy type pokemon: Pikachu, Noivern, Goodra, Talonflame, Hawlucha, Greninja<-->Ash-Greninja
Also when you look at Ash's team these are the types that are on it: Electric, Fire, Flying, Fighting, Water, Dark, and Dragon. That's actually pretty diverse.
Compared to the Sinnoh team which had these Weaknesses:
Number of pokemon weak to Ice-type attacks: 4
Number of Pokémon weak to Electric-type attacks: 2
Number of pokemon weak to Flying-type attacks: 2
Number of pokemon weak to Water-type attacks: 3
I do think he will still use reserves regardless of him catching a 6th capture or not.