You'll pretty much always want to have engineers with it no question, how much is up to you but I'd say at least 5. As for other units it is really flexible and depends on the situation, but if your enemy has air be certain to build reavers with it.Thank you! You advice was really helpful!
Just wondering, if I were to pair up with another player, and I was using Scarabs, what units should the other player build?
That again will entirely depend on the situation. Assuming your teammate doesn't build scarabs of there own, they need to focus on building an army to directly take on the enemy (like AA if the enemy builds air). If your teammate keeps his army with you they shouldn't need to worry about base destroying units cause the scarab can take care of that, but if they don't stick with you it again entirely depends on their situation.
Long story short there is no direct answer to that question, as it depends on too many factors to answer here, such as whether or not the teammate sticks close to the scarab. Assuming the teammate DOES stick near the scarab, your entire teams strategy will likely mold around the scarab and its defense.