So you take a Tyranid allied attachment just to be able to have 4 elites ?
Nice trick, not sure it's legal but if it's ok with your opponent then why not !
It's an excellent 1750 pts list, really straight forword. You do something that have been tested countless times and it's really good. 4
MC's in a small list can really wreak havoc.
Why the lone Zoanthrope tho ? For another synapse in the army ? For warp lance ?
A lone Zoanthrope never really do much by himself, warp lance is nice but it's not worth 600 pts
IMO.
Personnaly, if I had to field your list, I would tweak it by removing 2 Hive Guards (one in each brood), the Zoanthrope and the toxin sacs/adrenal glands on the Tervigons and I would put another
MC in the list. For those 200 pts you can have a Trygon. A Trygon can deep strike and make a subterranean assault wich is tactically very good. Also, you won't mind if he fail his synapse test... he gets rage. I find it worth it to exchange 2 S8 and 1 S10 shots for a
MC that can open anything that's armored and destroy most of the infantry units that are out there.
Also that way you don't have to take an allied detachment... your list will be legit.
But that's a personnal preference, your list is already quite good as it is.