While Protomechs are nominally organized into Stars of 25 Protos (when deployed in monogamous formations), the use of "Star" and "Lance" as descriptors for the formation bonuses is a bit misleading. It is generally less of a headache to simply call the 5-model Point of Protos its own formation, and have the remaining four 'Mechs in the Star be of a separate formation.
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I agree that it's "less of a headache" to do it that way. However, as a player of both Standard and Alpha, I would encourage anyone fielding Protomechs to strive to build their formations as complete as possible. This typically means 5 protos, 2 vehicles, or 1 Battlemech or Infantry element for a single Point when establishing a single Star.
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Using the Formations out of the Companion or Campaign Ops? Like aerospace units and infantry, Protomechs require a little more finesse to play effectively. Below are some of the things I have observed over the years fielding them and some of my experience using the Formation Bonuses with PMs in my mixed Stars/Novas.
"As the unit-count goes up, certain formations become less ideal."
Specifically those formations that pass out individual SPAs such as Assault [Multi-tasker or Demoralizer], Fire (any), and Command. The "individual" tends to get lost among the swarms, so it can get very confusing and frustrating late in the game as your elements start getting removed from the board or too far spread out to determine who would be the most effective holding onto that lone SPA.
"As the unit-count goes up, certain formations become more ideal"
Specifically formations where a percentage of the group receives the listed SPAs. Striker (Speed Demon), Recon (Forward Observer), and Pursuit formations have proven VERY effective, especially the latter which I use almost exclusively: I go from bringing 3 Bloodstalkers per Star (5*0.75 = 3.75, round down) to bringing 18 (25x0.75 = 18.75, round down).
[Math check: the total number of Bloodstalkers in a 25-unit formation is no different than 6 Pursuit Lances where 3 units from each lance receive the ability; the only difference here is diversity (in the 6 lances) versus repetition (protos); late in the game, foe assignment favors the Lances]
Personally, I've never honestly fielded more than 2 Point of Protos in the same formation at any one time. [Gorgon and Minotaur 3s are both "missile boats" and put in the same Fire Star... which was silly because only 2 units could ever receive Oblique Attacker!]
Since I abide by my group-instituted "Limit 5" Gentleman's Rule (to keep the cheese-heavy Swarm rules down), I have developed and successfully implemented "Mixed Star" formations utilizing 5 Protos, 2 vehicles*, and 3 battlemechs*. The Total Unit Count sits neatly at 10 for the formation, so a Pursuit Star only sees up to 7 Bloodstalkers (typically all 5 protomechs and both vehicles, but it varies based on games and gameday), while a Recon Star - if the conditions are perfect - will see 10 Forward Observers calling in artillery and other fire support... something very useful when playing Battalion/Cluster sized games or for campaign tracks.
* If the mechs and vehicles have the "OMNI" special, I also allow "Mixed Nova" to include a star of mechanized Battle Armor per the rule for Nova formations. The Infantry still do not actually add to the total units in the transporting formation (count remains 10 for that particular formation), but if the BA qualify for their own Formation individually (such as the Anti-Mech Lance in Campaign Ops), they may be assigned that instead.
Hope this helps.