Huh. Funny enough, I have been considering making it so you can recruit overleveled demons, but not summon them until you reach their level. (You could still, if this happened, Copy Ability, Fuse them, etc. Only Summon would be locked out. Also, such a demon would not gain levels until you caught up.) The restriction would probably be waived for uniques, since capturing those is usually pretty difficult, especially if underleveled.
That said.. while I can understand the broader point when the level difference is large, as it can be with say, Shoggoth in the Anomaly, 2 levels isn't that big of an edge: only about 15% or so. It's not nothing, but it isn't the +30-40% you can get from the others.
Unfortunately, the specific method you propose would be pretty tricky to do. It would be difficult to filter encounters based on level at time of spawning without some unfortunate consequences largely due to the fact I never planned on doing that when I designed them. Some zones would be reduced to almost no encounters depending on how tight that was adhered to. This could be fixed of course... by basically re-designing all the encounter charts.
Tell you though. I've been meaning to open up a new forum section where I basically post about what's on my mind and see what everyone thinks about it. Sort of a Suggestion box in reverse?
Lemme see if I can remember how to make a new forum and get that opened up, I'll make this one of the topics in it. I've heard other players (wizarrrgh most recently) make comments of a similar nature to this before, I'd rather discuss it in a more visible place.