Found out where I remember it from:
The Zone Mortalis Assault Mission
Scoring Units
In the Zone Mortalis assault mission, units drawn from either
the Troops or Elites allowance count as scoring units where this is called for by the mission. A unit may only ever claim one objective at a time.
That's in the Zone Mortalis expansion .PDF on Forgeworld's web page. Whether it made it into the version in Betrayal, I'm not sure.
An infantry tercio is 1-3 sections strong and the only real restriction listed is that if you want to field an Auxiliary Flamer Section you MUST field an Auxilia Lasrifle Section first. But other than that, nothing of that sort, otherwise Solar Auxilia players would be in deep gak when they need to spend 400+ points to get their minimum of two troops choices on the table in regular games.
Fair enough. I'm not sure why I was convinced it was 3, no more, no less, but I was. If it's not, then no problem.
There might be an argument for a small tercio of 1 Lasrifle section and 1 flamer section then - the flamers will be bloody horrific in a close quarter action.
Aside from the medics every model in this army is immune to the extra rules that make certain weapons rending and whatnot.
Only the rending, not the whatnot.
Cold Void & Poisoned Air is the rule which makes you rending if S4 or more, and Void Armour is immune to that (as are terminators, breachers, and squads who've taken Hardened Armour as a Zone Mortalis Stratagem - i.e. most units you're actually likely to see!).
Firestorm & Shrapnel is the one which gives blast & template weapons shred. This still applies - however Hardened Void Armour (which your veletaris get) does give you a reroll on your 4+ save - the rerolled save should more than make up for the rerolls to wound. The 'scatter stops when you hit a wall' does make massed frag missiles scarier against a squad in a choke point, though, so a small squad of siege tyrants could put down enough wounds to kill even veletaris.
Be wary of any AP4 blasts and templates, obviously! One of the scariest things you could see is a Havoc unit armed with heavy flamers.....
The board is usually set up in such a way that not all double wide corridors are connected, making it quite a chore for larger models to get around the complex.
Indeed - that's the normal approach and recommended by the rules, I believe. A dreadnought can't "go a huntin' marines", but if it can park itself on an objective in a room, it can be a serious obstacle to try and get past....