As a corollary to what Calgor Grim said bare in mind that putting in extra enemies strengthens all the enemies in the group.
So if said Bloodthirster had a Enemy Points Value of 100 and the Bloodthirsters had a EPV of 30 the total strength wouldn't be 130 but something more like 200.
I have noticed this is particularly true of hordes. PCs will often be getting their turn in before a horde. If they are only facing one large horde then they can often put down enough pain to cripple if not outright destroy it. However if you pit them against several hordes of equal total value it will be a far more dangerous proposition.
If you put in an encounter with Hordes Vehicles Elites and Masters this will be really quite difficult for the PCs. The KT great strength is its flexibility. By giving the enemy a mixture of resources you are suddenly giving them that same felxibility.
This is partially because there is a maximum damage bonus a horde can do which is reached once it goes over Mag 20.
For me combats can be split into two kinds.
Combats I know the PCs will win. These are there for plot reasons, or because it would make sense.
For example if the PCs are striding across a battle field it would make sense that they would encounter some enemy opposition even if said opposition is just a platoon of Fuedal World militia. Such encounters help to remind the PCs that against most foes i nthe Galaxy they will triumph.
Or for example say they are rescuing a Bishop from the clutches of some xenos worshipping cultists the actual rescue might not be the climax of the scenario however it would make sense for the PCs to fight the guards even though the combat might be over in a couple of rounds.
These combats you don't need to think too much about. Just allow the PCs to slice through the enemy and if necessary have remaining enemies flee.
Combats there to challenege the PCs. These I think should be 'crafted' in advance and be soemthing of an event. Consider terrain, motivations reinforcements. Really work out what will work and what tactics the enemies will use. In particular striking that fine line between tough horrible and unbeatable should be considered.