Try to play 4 rangers and 2 Wardens for the mid game, or 2 Rangers, 2 upgraded units, and 2 wardens. It's unlikely you'll have perfect units, so use whatever you can find until you find all your core units.
Additional comp tips
Statikk Shiv will be more of an early game item that will help you win streak, while crit items will make your late game strong with Twitch. Find a good balance between these two carries to help you win the game.
Leona and Karma can also be great units for your team before you are able to find all your core units, as they provide great frontline and dps with Karma's ability.
If you find yourself needing to add in Mystic, you can use Karma + another unit and take out Malphite instead.