Tervigons are HQ - not troops. I haven't played with them much, so I couldn't really comment effectively beyond noticing the Initiative 1 danger of Space Wolf psychers. If you look at the Venomthrope models, you'll see they can effectively hide behind a Tyrant model, only real concern being blast templates.
As for the others, 50 points of Guardsmen will kill 37.5 points of Termigaunts before they can assault (one round regular fire, one round rapid fire). They'll kill 45 points of Hormagaunts the same way. They'd kill 47 points of Genestealers. Given low cost, and overall utility in cheap squads that still hold firepower, there's not much reason to not take 60+ regular Guardsmen just for bullet shields. You can expect similar cost performance for shooting small bugs from regular Tac marines. If you're playing in a forest with plentiful cover saves, the numbers would change, but I'm not fond of the idea of losing 75% to 94% of my troops before they hit the enemy lines - usually need a little more to be viable.
Quite right on the scoring units, though. I play a lot of annihilation, and I'm not entirely used to enemies being alive beyond 5 rounds. You can contest with the big bugs, but it wouldn't hurt to throw in two minimal squads of Gaunts if needed.