Short answer, you don't have level -1 creatures. You would use less than 6 level 1 Skirmishers. Remember that these are templates and you don't have to do exactly what they say to use them.
In this example, a party of 5 PCs in a standard encounter would expect you to spend 500xp on monsters, traps and the like. In this example you can buy the following and have the Battlefield Control feel to the encounter:
- Goblin Hexer Level 3 Controller Leader for 150xp
- 3 Goblin Warrior Level 1 Skirmisher for 300xp
- 2 Goblin Cutter Level 1 Minion for 50xp
You could also drop the 2 Goblin Cutters and have another Goblin Warrior if you wanted to bump the XP reward slightly and keep it close to a standard encounter. There is plenty of wiggle room.
In principle, start with the key feature (like the controller, or the mix of types) and spend your points on as balanced a remainder as possible.