excuse me? the guy asked for advice on how to make a swarm army work.
Malanthropes with tough Synapse/Shrouded bubble are basiclally a key part of that concept. I know because its very similar to what I run. I dont actually get why you're all prissy about cost, I never said they we're cheap or anything of the sort, and their are plenty of conversions view able on the net that make good looking malanthropes without the FW cost and shipping.
as to toxin sacs....they aren't worth it at 3 pts per model, 8 pt T3 6+ save models are not good. You are better off taking those pts and either putting it into more bodies or using the pts to buy bugs that either will kill things (after the Horms have pinned a unit down) or allow you to boost the gaunts (using psychic powers and or the swarm lord) though that is harder these days with the lack of synergy in our codex.
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Theres solid justification for malanthropes to be present during any post invasion list really. because the Nids don't exactly wait to start feeding, they just get right too it.
However if you are dedicated to not running Malanthropes I would recommend this set up instead.
2xZoans brood
Venomthrope brood
The venomthrope will provide the shrouded buff that allows your gribblies to work and the zoanthropes provide the much needed synapse along with the ability to try and crack armour so that your little guys can get into the guys inside. Furthermore each Zoansthrope broodwill get a roll on the nid psychic table, giving you a better chance at getting the Buff/De-buff powers that will allow your gaunts to hurt things / survive.
Finally, A third way to use the list is to run the Endless Swarm Formation, you have the bugs for it and what it means is that your units will return on a 4+ after they are killed. And 6 units of 20 are easier to maneuver than the units of 30