@Charles Tottington as always with new rulebooks I keep dipping in and finding more nuance on re-reading... like you I do love a bigger game and this ruleset will allow it; I just try to find the focus I like in a game and go from there. Again, I haven't got the pdf so I don't know if it's somewhat "redacted" from the full hardcopy version, but you can definitely add more than one sqaud of five (either co-op or just playing multiple sqauds yourself) earth troops but then you need to add more alien troops/monstrosities to balance things.
With the alien troopers you get options for ranged and melee opponents, plus two different drones and an enemy commander; these come in at various force levels to create a six-game micro-campaign (this starts on Page 78 of the book) which is detailed with terrain maps too; by the end of things your little dudes will be up against potentially eighteen FORCE troops... and you can add to this if you use more than one squad of five although I think you'd need to up the suggested 3x3 for this!
This aspect will be my focus, with some "boss fights" chucked in for flavour, As you'll see from the pic I'm using some old Antares Algoryn for the troopers, ranged pretty much as they come, then some melee troops with Lightning Claws (I just orderd some Lightning Claws!), but termagaunts would seem a cool bet too here with hormagaunts for the melee element 😎
I'll use some Antares Ghar Battle suits for the drones, and then stat up the skimmers in line with the exitsing rules. "Monstrosities" will be various alien beasties from my existing collection (including hormagaunts, love these!); with a little fettling of the trooper stats you could create multi-figure alien "squads" too, and it seems to me that various of the larger GW Xenos would be prefect for this game as "bosses" 😍
That's where I am right now, amassing my forces and building/re-purposing some figures before I get stuck in to my first proper game... and aye, there's the starting fun right there 😍