It depends on league, tournament, state, etc., requirements. For example, here in Omaha, for league and weekend tournament play (just a regular tournament, and I'm speaking from slow-pitch), we're allowed to wear shorts, even if our partner has pants on. It's an individual preference.

As for the state tournaments here in Nebraska, we have to match our partner in regards to shirt and pants/shorts. If they're wearing shorts, I can wear shorts. If he/she's got pants on, I have to switch to pants. Same with the shirt, as some city leagues apparently use the navy blue shirts while most use the powder blue.

Like I said, it depends on the locality, etc. etc. As for determining who matches, etc., you should have that information from the league you're working for. Here in Omaha, we have an UIC and (I believe) 3 deputies, as well as the league office President, who can make that determination if we are in uniform or not. They're not so worried about matching for league play, more so about being in appropriate uniform.