I can't speak for Outlaws specifically, so I don't know if it's considered on the harder or easier side of the scale, but most likely you could run as is and find that it wouldn't wreck the game. Particularly if your players are newer or trend toward the less tactical side.
If you end up finding that your players need more of a challenge thrown at them, my recommendation (mainly for simplicity) is to add +1 minion to any group of enemies lower level than the player. If it doesn't make sense to add an extra enemy, consider the Elite template (i.e. +2 to all stats) or even just going halfway and adding +1 to all stats. Finally be careful around any boss monster. Often they're already at a level that would make even +1 attack or AC a major nuisance, so in these cases I would seriously consider just letting the boss fly as-written if I couldn't fit a minor minion.
Pure encounter math-wise, 5 PCs is +25% Player Power(tm), so adding 1 to a group of 4 enemies keeps the threat rating identical in theory, and for any larger group shouldn't unbalance anything. For a solo enemy that's not in a boss-position, Elite might be a major threat boost (+50% power on paper) so consider +1 to stats instead, unless the solo enemy is nearly a non-threat. A solo enemy that's higher than party level is probably pulling significant weight already, so maybe a few extra hit points if you want to make the fight last a little longer is best, but if you need a minion, something that's 4 levels lower than the original monster should be about +25% power
What Sibelius said, but I want to reinforce his warning about elite templates on a Severe solo boss, because that, in effect, makes it an Extreme encounter, which are TPK traps. Really better to just leave it as it is or add a minion.