Hold on. So a 1st level of 5 Evil NPC party with healing surges and actions points is worth the same XP of 5 Kobold Skirmishes?
Reebo
If they're the same level as the party, party members are going to die, it's likely a 50/50 TPK. If they're lower level than the party they should be able to figure that out quickly, maybe even before combat, and flee/surrender/whatever.
Short answer, DnD doesn't work for fair fights, don't run 5 fully powered NPCs against the party. Run 1 with minions or whatever.