You can have an administrative leader and a different person who makes the calls during the match. In my experience the medic and/or his pockets are in the best position to do this since they are the core. I played soldier for UGC HL just this season and while I was the most experienced on the team and tried to mentor them, I couldn't make calls from a roaming position. In 6v6s I normally play pocket and either my medic or I make the calls.

I only ask for people to run certain loadouts in certain instances. For example, if snipers are going to be a problem I may ask the pyro to use the flare/detonator. I may ask the scout to bonk out on the initial push. I may request the soldier to use a buff banner. I will sometimes ask the medic to swap to a kritzkrieg, but this is a decision I would like to leave to them since they are in charge of tracking uber advantage.

But these are mostly map or situation-specific instances; I don't think its a good idea to try and micromanage loadouts since each player has a weapon set they are comfortable with and will change out on their own during the match as the situation changes.

Personally I find that one team leader is not enough for HL. I try to mentor and make calls for my team, but I don't have the time to deal with scheduling and rostering and subbing issues. In-game is when I do my thing.