I already did this in my campaign. My solution is really simple:
Open up your Monster Manual to page 277. note the writeup of Githyanki "provided to help Dungeon Masters create nonplayer characters".
Open up the DMG to page 187 and follow the NPC design steps to make some Githyanki NPCs of the appropriate level. Githyanki make good warlocks, staff wizards, swordmages and artificers, so consider those options.
Figure out why the githyanki running the camp were on the lower end of the githyanki power scale. In my campaign, the PCs encountered a gith outpost that worked sort of as a remote post you exiled troublemakers to, like Siberia for communist Russia. You sent all the losers and incompetents to the Prime Material Plane. In your campaign, the same might be true: guard duty for a mining camp is certainly less glamorous than riding a red dragon into battle on the Astral Sea. So maybe the guards here are just the weakest that the githyanki have to offer (they underestimated the elves and humans, due to racial arrogance).
You still might want some servitor race helping the giths, if only so the githyanki could be two or so levels above the party's level. Difficult, but not impossible to overcome. Meanwhile, the githyanki have slave hobgoblins or warforged or something working for them who are the primary cannon fodder race. (I know all the warforged in the MM are level 4-6, but you can once again make some warforged NPCs or monsters of the appropriate level).