No, you can set say a 50% chance of getting a militia group with heavy and another 50% group with light frigates, but that really means 25% you get nothing, 50% of the time you get one and not the other, and 25% of the time you get both. Its all probability as there is no way to check if a group has already spawned on a planet or not.
I suppose you might be able to get some crazy ability that spawns in just one or the other, but that might be more trouble than its worth.
Edit: Actually the two of those could work out. Give a 50% spawn to say the light frigates, as well as a 100% spawn to a generic frigate with 1 hp. Give that generic frigate to the pirates or anything else that is hostile to all players and local militia. If that frigate survives say 30 seconds, obviously the light frigates did not spawn so have it trigger an ability that spawns some heavy cruisers or whatever you will.