Well also consider that while yes, your allies are sapping XP, they are also going to be absorbing some of the monsters that generate that XP. Getting a full 900 XP from 900 XP worth of X-Ds isn't worth much if that 900 XP worth of X-Ds kill you. I haven't fully built the Command Center fight yet so I'm not sure how much XP it'll actually be worth - it might be enough to push you over. The fight in the research labs I'm pretty sure won't, so that might be another reason to do the Command Center first.

As for the Stables fight I can re-check the math. Here:

Spoiler: XP Calculation
Show

Allies: -350 XP
  • Tygrin (Bloodied) - Level 10 Skirmisher (-250 XP)
  • 2 x Archangel Crewmen - Level 1 Artillery Super Minions (-100 XP)


Enemies: 850 XP
  • Reaper - Level 4 Elite Soldier/Brute (350 XP)
  • Sectoid Trooper - Level 3 Skirmisher (150 XP)
  • Sectoid Centurion - Level 3 Solider (150 XP)
  • 4 x Ichoroids - Level 1 Controller Minion (100 XP)
  • 4 x Sectoid Grunts - Level 1 Skirmisher (100 XP)


850 - 350 = 500 XP

Yarp, 500 XP.