1. Grunt Squads are 1 Spec per every 4 non-spec troopers.
2. Elite Squads are 1 Spec per every 3 non-spec troopers.
So, with squads that have multiple Specs available, you must have 4 troopers for each specialist, or 3 per with elites.
In order to get the 2 HMG's, or 2 Snipers, or 1 each, you must have 8 troops (6 for elites), to get 2 specs.
Hope this helps.