I love the conversions, but would of thought keeping things just as tyranids for mounts would of worked a bit better than adding in some other stuff. So for stonehorn gone for either the Toxicrene or the Mawloc/Trygon and for Mournfang going with Tyrant Guard, Raveners or Venomthropes.
Would of been cool having all the Tentacles everywhere.
That is my 2c and if you like your Idea stick with it because so far it looks really good, and the colours really look good for the Jungle theme, like the Light green markings on the Shell, maybe look at doing a few more.