You can't set the levels for custom critters (yet). (Custom groups inherit the level ranges of their contents.)
What you can do is add a singular (even optional) critter to the mission to force the mission as a whole to spawn in a given level range. For example, if you add a Hellion boss, that forces the mission to cap at 14* no matter what the general enemy type for the map is.
Similarly, adding a Hellion hostage also forces the level range of the map. This is non-intuitive since the Hellion doesn't actually fight or take damage and RP-wise could conceivably just be in the wrong place at the wrong time, but that's how it works.
* Assuming Hellions cap at 14, I forget what the number is.
And for a while things were cold,
They were scared down in their holes
The forest that once was green
Was colored black by those killing machines
Sorry if this has been asked/answered a bajillion times. I didn't see it on the Paragon Wiki and haven't seen it slogging through threads on here.
Can you set a level on custom mobs? I'm making a low-level mission arc but my custom mobs are all level 1-54. Next mission in, I'm adding some Hellions so I think they'll cap it at level 14 or 20 but I'd prefer to be able to say the entire arc is for levels 1-14 if at all possible.