I don't think the model count is too small. All those multi-Wound models will help to soak up the majority of anti-infantry fire, and opponents who direct enough anti-tank fire to Instant Death your Warriors will be ignoring all the MCs running towards their lines.
The Tyrannofex seems to me like an investment in not just firepower, but the assurance that it's going to be around for the whole game. T6, 6 Wounds and a 2+ armor save, combined with the fact that it's not charging forward to eat you, means your opponent will probably ignore it rather than wasting time and firepower. You could accomplish the same tank-busting for fewer points by taking a Zoanthrope Brood in a Mycetic Spore, but their lack of resilience makes them priority one targets and they probably wouldn't last all that long.