Actually mini-mob of orks isn't bad for an early encounter. Even better if you start with a couple, and have more around the corner to "join in the fighn'", allowing you to bump up the numbers (or not) based on how well the party is doing. An ork's (un)natural toughness and good strength make them solid, but not overwhelming, enemies. As an added GM bonus, they're gear is all but useless to the PCs, reducing loot issues.
Don't be afraid to use "narrative background combat". Have the PCs fight a Nob and a half-dozen orks, while their regiment of redshirts fight the Tide of greenskins in the background.
When fighting organized militaries, heavy weapons and combined fire can go a long way to even the odds, as any
Imperial Guard Astra Millitarum player can tell you. Your players will (and should) be outnumbered when dealing with minion enemies, then occasionally fight a big-bad of the 41st Millenium, of which they're are plenty of every power level. (Eldar Autarchs, Ork Kaptans & Warbosses, Chaos Space Marines, even the lowly Ogryn)
My biggest advice for balancing combats, is to first look at what 'average' PC weapon needs to roll to hurt the enemy, and what the 'average' enemy weapon needs to roll to hurt a PC. 2nd, don't be afraid to have enemies do something less helpful - like move to different cover, reload their weapon, hug cover, attempt to close to melee to avenge their brother, or break and run. You can often freehand adjust the effective difficulty of an encounter by making the enemies act more 'naturaly' and less 'statistically appropriately'