I'm running an adventure for 3 level 2 (at this time) players. They've come to a town that is overtaxed and starving, and the government hasn't made a move. It turns out that most of the leaders are being controlled by Mind Infestors, small, squid-like creations of the Mind-Flayers. They latch onto the backs of people's heads and control their actions. I'm having the players learn more and more about the conspiracy each session, like episodes of a TV series. When the players finally unravel the mystery, I'll have them enter either the blacksmith shop or the government base, both entrances to the makeshift mind flayer colony. This is where my problems come up. The players will be level five by then. In the DMG (while describing players level 5-10) it states the following:

These adventurers venture into fearsome wilds and ancient ruins, where they confront... crafty mind flayers...

My question is this: Are mind flayers on the higher side of levels 1-10, or the lower side? If I can't use them, I'll probably upgrade a commoner so it would be a good enemy to stuff the majority of the lair and/or be an enemy on a random encounter table. Then I could have a mind flayer boss at the end of the dungeon and promise more of them later.