In Garrus' Recruitment Mission in ME2, if you allow the Heavy Mech go through the bridge, he will incorrectly say that "Heavy Mechs are in the base"
So, was there plan in having more than one Heavy Mech in this mission, or was it just a typo?
I do not know for certain, but do remember that initially Mass Effect 2 was going to be very different.
There are even recorded dialogues from Kelly stating that Zaed and Kasumi want to talk (presumably they would be recruited in a different manner and only state the necessity of their loyalty missions later, just like the remaining companions) .
In the original Mass Effect 2 concept you would be able to recruit people in different orders. I don't know if the content would change based on your class or background or even if all dossiers would be available from the get go but all characters were designed to participate in all of the recruitment missions and their disabled files are still present in the game. On the pc version, with hacking devices it´s easy for anyone to edit parameters and experience the first set of missions (Garrus, Mordin, Grunt and Mordin) with the second wave of characters.
It seems that the creators were very uncertain during production regarding how many disks the game would occupy, how the content would be distributed and in what proportion the content would become available as the player progressed. There are several recorded lines from the illusive man releasing a varied number of dossiers. It´s even possible that the number would vary depending on how many dlc characters you had purchased and installed.
My best guess is that the xbox 360 version being on DVD and not bluray (nor demanding that people install it on a hard drive) ruined the experience because it would require that people switch disks constantly. Making the game unavailable for xbox unless a hard drive was used would probably irritate microsoft and drastically affect sales for bioware.
Too bad, because some scenes are really cool, like taking Tali in order to recruit Garrus. The material is worth watching and is available on youtube.
This kind of device, along with console commands and several kinds of mods are one of the reasons why I love pc and don´t game on consoles anymore. These alternatives can increase the longevity of the product by quite a lot.
With that being said, it is very possible that during the design of the game, while trying to determine the level of strength of the enemies they did consider having more than one mech attacking at the same time. Maybe it would scale up based on the player level. If Garrus was one the first recruits, then just one mech. If he was the last, maybe two, or even more. It´s worth noting that Garru´s loyalty mission actually involves two heavy mechs attacking simultaneously. By the way, during the party in the citadel dlc for me3, Garrus states that he always wanted to purchase a heavy mech for himself.
As much as I like ME2 in the form it became available, the fact is that the game was envisioned to be much better. Maybe in 15 years they will release an expanded version that will allow what they could not do initially.