I wouldnt play a mega large battle
Actually I was thinking of replaying Camerone as one of my new projects for a wargaming convention.
I would not play the square and retreat to the hacienda. I would play the defense with focus on the hotspots.
Gateway
breach
ranch house
The plan was ( I hope my math is right ) 6 units of 5 figures each ( 30 miniatures representing 60 men ) plus three officers ( Leaders )
The Mexicans have 6 units of 10 figures each ( 60 miniatures representing 120 men ) - If you need more figs you can recycle destroyed Mexican units. The Mexicans can use ladders or fire or a ram.
The Legion will ignore suppression because they can not run and will fight to the death. The Legion is in hard cover as long as the Mexicans are outside the perimeter.
The Legion are all Veterans and will always have initiative although I think no Mexican unit should be better than trained. Maybe one Unit of Veterans. But that has to be tested.
Victory conditions are calculated on a victory bar. Which is affected by killed officers, killed units, objectives, number of rounds played, water and ammunition supply for the legion.
In our last stand games we had volley fire tickets ( cards that were played against the enemy representing heavy defense fire causing extra suppression - to break shaken units - but after each card the ammo level dropped )...
Anyway what I experienced in my wargaming years is that you can adopt almost every ruleset for the scenario you like to play.
If you are playing multiplayer games you should not use IGUG rules. That gaming mechanism takes too long and will tire gamers during the scenario... But you can use it.
Particular games need some arrangement and special features like the ones I tried to explain above.
What rules will you choose for Gandamak ?
We should exchange our ideas here. Its helpful and inspiring
Cheers
Björn