You can request/write any pairing you want as long as it involves at least one of the requested characters. Pairings don't have to be made up of just requested characters.

So....

PumpkinKing's favorite characters are Doritos and Moss Covered Unicorn. They ship both Doritos/Moss Covered Unicorn and Doritos/Monster Munch, but while they'd enjoy Doritos by themself or Moss Covered Unicorn by themself, they wouldn't enjoy something that was just Monster Munch. So PumpkinKing opts not to add Monster Munch to their list of selected tags, cause they don't want to risk getting someone that was really excited to just write Monster Munch, and instead adds their love of Doritos/Monster Munch to their optional details. They also decide that they kind of love Doritos with the unnominated Candy Corn, so they can add that as an optional detail.

Short version: As long as PumpkinKing understands that they might just receive a single requested character they can still include any prompts for groups or pairings that include at least one requested character or give a blanket approval.

Does that answer you question? Also, I wish I had Doritos now.