My goal is to give each player a role in a "military" as a way of adding something to the game that the players may not have experienced before in a D&D game. So they may be given skirmish missions from a higher-up, but I want them to be in charge of the team (squad) provided to them. I want them to feel like they are in charge and are filling a special role in the squad that other members of the group can't provide as well.
Are the PCs the only members of the squad?
I want the more warrior types (fighters, paladins, cleric, etc) to be in charge of the squad and would be the one reporting to the higher-ups.
This is one of those areas where D&D's simulation falls. The person in charge is, at minimum, an experienced and charismatic person. Hopefully intelligent too. These aren't things that are associated with D&D fighter-types. (That's why I mentioned warlords above. In my current 4e game, where I'm a player, we acknowledge our warlord as leader. He has Int 10.)
In other words, PCs aren't competent enough (within the rules) for the roles you're trying to give them, at least without introducing a series of often poorly-balanced non-core rules. (A marshal might be a competent leader, but they're pretty wimpy in actual combat.)
I'm a big fan of the really old Chinese classic, Romance of the Three Kingdoms. Drop the first three words, that's not important, or replace "Romance" with "Book". It's all about a massive civil war. All three faction leaders were nobles (think aristocrats), each had a group of "Tiger Generals" (or "Kings of Wu" for the southern group) who were generally charismatic badasses but had varying levels of intelligence*, and each were usually assisted by a very intelligent adviser who was usually
not a badass. Perhaps you could replicate that.
*I'm picturing fighter/marshals. There were some who were clearly better at fighting than others, and some who were clearly better at thinking. I'm not really sure if there were any who were
really good at both, although Gan Ning's bold leadership certainly made him
seem very intelligent. Even if all he did was roll dice to randomly select a strategy!
Another couple of examples are English and Roman. During the Hundred Years' War, the highest-ranking commanders got their position by birth and commanded numerous contingents from their duchy. Each English contingent generally consisted of ~100-120 soldiers, of which one quarter were knights (they often fought dismounted) and the rest were archers. Archer leaders were called "centenars" (lead 75-100 archers) or "ventenars" (lead 20 archers). These contingents would be led by a noble (a count for instance), who was generally a fairly badass warrior, and may or may not be a competent field leader. Fortunately for the English, their high-ranking leaders were either good leaders or at least listened to their advisers. I couldn't find good info on other types of soldiers (they couldn't all have been just knights and archers).
In such a system, one of the contingent leaders could hire mercenaries (the PCs) to act as scouts and so forth, and this noble must in turn report to a higher-ranking noble. This noble is responsible for them, but what ranks these mercenaries are given isn't really relevant.
And finally, the Roman system. Romans marched in units of 10, of which two were noncombatants and possibly slaves. 10 units gave a century, led by a centurion and an optio (basically really high-ranking NCOs, because they were promoted from the ranks) and a legion would have a senior centurion, the primpilus or primipilus (first spear), who was both a badass and a really good commander, plus the aquilifer ("eagle carrier"), whose job was to protect the legion standard (aquila) and was the legion's most badass person. The aquilifier did
not need to be a good commander, and that's a great place for a high-ranking fighter to go to, although high Charisma helps rally troops to protect the aquila. There were essentially noble officers (legatus, the guy officially in charge, several tribunes, who were usually very young noblemen) and weren't really expected to be good at war. Sometimes a governor led the legion instead of the legatus, or led several legions (Julius Caesar did this).
A group of PCs isn't even the size of a 10 man Roman squad though.
Fighter
Cleric/Sorcerer
Beguiler
Bard
Since you want the fighter to at least nominally lead the unit, I need to know more. Do they have decent Int and Cha scores? Do they have noble birth? Tactical feats? Because if they have neither, they cannot effectively lead in flavor terms. They can only be the badass.
What is the cleric/sorcerer "specialized" in? Domains? Favorite spells? That's a pretty suboptimal combo too. If this PC really drops the bomb on enemies, I would give a title such as "War Wizard" or "Senior Wand". But there's not enough info here.
Beguilers have a wide array of out-of-combat spells they can use, such as Charm Person. (I refuse to see such a spell as a combat spell.) So this PC could make a good infiltrator or "Covert Ops Agent", but you need an old-fashioned title for that. But again, you know your PC better than I do. A beguiler who spams Confusion is something else entirely.
A bard is much like a Beguiler. They have actual combat leadership abilities (high Charisma, the ability to enhance their allies, social skills for out-of-combat) and if they're of noble birth all they need is a sword to be a standard of leadership and you now have a party leader. Even if they have low Int, that just means they need to lean on an adviser when prepping for battle (I'm looking at the Beguiler) and then follow the "script". Keep the troops focused. Said bard would probably have a title like "Lieutenant" or "Captain" and will probably customize the title to better suit them. "Almost-Major?"