FYI, from the source material:
(S8.331) There can be no more than one size class 2 ship (dreadnoughts, most CVAs and SCSs) in the fleet. Size-2 ships never appear with less than three other ships, two of them from the same race as the size-2 ship.
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(S8.46) TUGS: No more than one tug (including LTTs) can be included. This does not count tugs carrying cargo pods (or self-defense pods, but no other type) in a scenario where the mission is to deliver cargo. This does not count tugs carrying troop transport pods (or self-defense pods, or hangar pods with GAS shuttles instead of fighters, but no other type) in a scenario where the objective is to deliver troops.
Full battle tugs are invariably size class 2. An LTT with a light battle pod is size class 3.