I think there a bunch of monsters that could have interesting larva/nymphs/yound. Off the top of my head, the grick, roper, and umberhulk could have pretty nasty larval forms.
The nice thing about this is that you get to use them earlier. So you don't have to just level them down, but you pop out a young one, and now your lower level PCs get to fight something classic.
Not to mention that it can be a great foreshadowing/make them nervous. If they just fought a bunch of baby grell, they have to wonder where the parents are.
Also, don't forget about alternate methods of reproduction for your aberrations. The mind flayer, after all, has tadpoles that are implanted into the skull, metamorphsizing into human. So a "Transform a victim" method of reproduction is possible.
Or go the Xenomorph route; they either 1) all inject targets (perhaps captured/paralyzed/dead targets) with eggs that incubate in the body, or 2) produce a secondary creature that injects the eggs (ala facehuggers). Route 1 works for carrion crawlers (they all ready paralyze targets to munch on, why not also lay eggs on them?) and route 2, imo, fits Grell. Simply because grell are freaking scary.
(Trivia: the inspiration for the xenomorph breeding in
Alien was inspired by the parasitic wasp that lays its eggs in a paralyzed insect.)