The random rolls was meant for non-player opposing forces, not player character forces. So you don't fight the same thing over and over. We're looking at the feedback to have a player "create your own" option though.
Fair enough, it is the only "create a force" section in the book so I assumed that was it. I am glad you are looking into a create your own idea.
Honestly if you just created a template for players to use like the one you have for all the merc commands in the book that would be awesome. Give them something similar to Faction Points called Command Points(CP).
For the creation rules (maybe on a point system? Negative quirks would be good here to balance them out), you just follow a format like the following:
1)Pick your origin: So you should have several options here. They can all cost CP, but all should have some benefits and negatives. Did the unit break away from their former faction? If so they now gain an extra enemy (that faction). Are they the personal guard of some rich, pompous noble? They have fancy equipment, but cannot get the
espirit du corps ability.
2)Pick your friends and enemies: limit it to one each. This means you can pick up some equipment from your friends for FP as per the rules.
3)Force composition: This is where things get a bit hairy, but hear me out. Everyone should start with a light rated company/battalion/regiment (the size should not matter for the purpose of building). This can be a vehicle, infantry, aerospace, or Battlemech unit. You can pay CP to add additional subunits, i.e. add a vehicle company. You can pay CP to upgrade those subunits to higher tonnage ratings. So I could pay points to make my Light Battlemech battalion a Heavy Battlemech Battalion, as well as purchasing a light vehicle company to go along with it.
4) Commands and subcommands: Generally player-made units should be limited to one command. However, paying a hefty CP cost can allow players to break their unit up into smaller commands under a general umbrella.
5)Command and subcommand abilities: This is pretty straight forward. Using the CP system, allow players to buy command abilities for their unit as well as paying points to up the skill rating of their various subunits so that they can add more abilities to said units. Also, remember the origins? That and force composition should dictate what command abilities you can and cannot buy for your units. For example, a company made up of Assault weights is not going to be able to gain "Hit and Run".
All in all, it should be realistic. If you want to play a legendary mercenary unit, play one of the canon units. Mercenaries are a dime a dozen, but that doesn't mean your unit cannot be fantastic as well.