Okay so you could have Rapidash hold a Grass Gem which would raise the power of Solarbeam by 50% the first turn you used it, both after that, no. Honestly I wouldn't use Rapidash, I'd go with something like Infernape or Darmanitan. Also, when you are making a moveset, covering weaknesses is great, but don't focus the moveset entirely on covering weaknesses, make it geared toward the specific role your Pokemon plays, and if that's sweeping, you don't always want to cover weaknesses, but any defensive counters, as well as getting the widest range of coverage possible (hitting the most Pokemon unresisted/super-effective).
