Sorry for the delay in response. This matter is rather tricky. Character requests are optional for participant to fulfill and participants are technically not obligated to include any of the nominated characters in their work as long as the character they do include belongs in the fandom requested and is chromatic.

We would say it's relatively okay to expand on character preferences in the optional details. However, just be aware that that even if you requested "any" for a fandom, you're not guarantee to be matched with someone who offered any. In fact, you might be matched with someone who only offer for "Character X" so we do ask you to keep that in mind and (continue to) be considerate to your potential assigned participant by, for example, stressing the "optional" and "any" part of the requests.

I hope that address your question.