I'd like to take a closer look at Poison/Electric, Fire/Electric, Fire/Fairy, and Ground/Electric, so I'm going to make a comparison between the four. Fire/Water has already been thoroughly covered. Let's begin by examining the typings defensively.
Poison/Electric
*0: None
*0.25: None
*0.5: Bug, Electric, Fairy, Fighting, Flying, Grass, Poison, Steel
*1: Dark, Dragon, Fire, Ghost, Ice, Normal, Rock, Water
*2: Psychic
*4: Ground
Fire/Electric
*0: None
*0.25: Steel
*0.5: Bug, Electric, Fairy, Fire, Flying, Grass, Ice
*1: Dark, Dragon, Fighting, Ghost, Normal, Poison, Psychic
*2: Water, Rock
*4: Ground
Fire/Fairy
*0: Dragon
*0.25: Bug
*0.5: Dark, Fairy, Fire, Fighting, Grass, Ice
*1: Electric, Flying, Ghost, Normal, Psychic, Steel
*2: Ground, Poison, Rock, Water
*4: None
Ground/Electric
*0: Electric
*0.25: None
*0.5: Flying, Rock, Poison, Steel
*1: Bug, Dark, Dragon, Fairy, Fire, Fighting, Ghost, Normal, Psychic
*2: Grass, Ground, Ice, Water
*4: None
I think that Poison/Electric might be too much for what we're trying to accomplish here, resisting nearly everything that would give Lucario and Latias trouble. However, I think it would also function too well outside of the core with the that typing. Ground/Electric doesn't resist Dragon, Fairy, Dark, or Ghost while being weak to Ground. It also resists Stealth Rock, so Latias' Defog utility is lessened as well. As such, I don't think either of these typings are defensively appropriate for CAP 18.
That leaves Fire/Fairy and Fire/Electric. The former has a useful immunity to Dragon, to which Latias is weak and Lucario is too frail to normally handle all that well, as well as resistances to the Dark/Fighting combo and Fairy attacks that give the core so much trouble. However, the latter resists Steel and Flying, which makes it a better answer to Flying types and Aegislash.
So, we should move on to offensive presence to best determine which of these typings is ideal. I'll be using shinyskarmory's list from earlier in the thread for this part of the comparison.
Poison/Electric
Resisted: Excadrill, Ferrothorn, Garchomp, Gliscor, Landorus-I, Landorus-T
Neutral: Aegislash, Bisharp, Conkeldurr, Deoxys-S, Dragonite, Heatran, Latios, Latias, Lucario, Mega Charizard X, Mega Mawile, Mega Venusaur, Rotom-W, Scizor, Thundurus-I, Tyranitar
Super effective: Azumarill, Clefable, Greninja, Mandibuzz, Mega Charizard Y, Mega Pinsir, Skarmory, Talonflame
Fire/Electric
Resisted: Garchomp, Latias, Latios, Mega Charizard X
Neutral: Clefable, Conkeldurr, Deoxys-S, Dragonite, Gliscor, Heatran, Landorus-I, Landorus-T, Mega Venusaur, Rotom-W, Thundurus-I, Tyranitar
Super effective: Aegislash, Azumarill, Bisharp, Excadrill, Ferrothorn, Greninja, Lucario, Mandibuzz, Mega Charizard Y, Mega Mawile, Mega Pinsir, Scizor, Skarmory, Talonflame
Fire/Fairy
Resisted: Heatran, Mega Charizard Y, Talonflame
Neutral: Azumarill, Clefable, Deoxys-S, Gliscor, Landorus-I, Landorus-T, Mega Charizard X, Mega Venusaur, Rotom-W, Thundurus-I
Super effective: Aegislash, Bisharp, Conkeldurr, Dragonite, Excadrill, Ferrothorn, Garchomp, Greninja, Latias, Latios, Lucario, Mandibuzz, Mega Mawile, Mega Pinsir, Scizor, Skarmory, Tyranitar
Ground/Electric
Resisted: Gliscor, Landorus-I, Landorus-T, Latios, Latias
Neutral: Clefable, Conkeldurr, Deoxys-S, Dragonite, Ferrothorn, Garchomp, Mega Venusaur, Rotom-W, Scizor, Thundurus-I
Super effective: Aegislash, Azumarill, Bisharp, Excadrill, Greninja, Heatran, Lucario, Mandibuzz, Mega Charizard X, Mega Charizard Y, Mega Mawile, Mega Pinsir, Skarmory, Talonflame, Tyranitar
Both Ground/Electric and Poison/Electric are ineffective against Ground-types that can give the core trouble, particularly both forms of Landorus. If we were to go with one of these two typings, Ice type moves would be the best way to patch up the holes in coverage. Fire/Electric struggles with two major threats in Garchomp and Latios; Ice is the single best type to deal with this typing's shortcomings. Fire/Fairy gets neutral coverage on nearly everything; Electric or Rock would deal with the few that resist this typing.
In conclusion, Fire/Fairy is the best option out of these four choices. Defensively, it can take the Dark, Dragon, and Bug type attacks that Latias dislikes while also resisting the Fire and Fighting attacks that Lucario can't handle. It's weak to Water, Ground, and Rock; Latias can sponge the first two types, while Lucario laughs at Rock type attacks. Its susceptibility to entry hazards also encourages the use of Defog by Latias, and Lucario appreciates having a teammate that can absorb incoming burns. Offensively, Fire/Fairy hits the majority of threats to the core for at least neutral damage. Aegislash and other Steel types get roasted by STAB Fire attacks, while Conkeldurr, Garchomp, and Latios are hit super effectively by Fairy STAB. The bulky Grounds and other Fairies that may give this typing issues can be dealt with appropriate coverage moves.