Vincedis, your army idea seems pretty decent. The only problem that I see is that the devilfish transports are expensive. I'm just tryin to look out for you, cuz I think it may be a waste of points (and money).
As we're both DE players, we're too used to the idea of cheap transports!
See, bring the fire warriors into a forward position, even under cover is just an invitation for assault troops to kill them in HtH. To them, you're just helping the assault troops out by bringing them in closer so they don't have to fly as far. Think of it from a DE standpoint: Your squad of wyches see the firewarriors come in closer. They go "oooh, goodie" fly their raider over, disembark, FoF, throw some plasma grenades, and wipe the floor with the firewarriors.
On the other hand, if the firewarriors were to stay further back, then the Wyches would be forced to fly under cover, and hopefully, make a mistake where you could unload your guns and destroy them before they even caused a problem.
Well, in the end, you'll never know until you try. I could be completely wrong, but i'm just giving you my humble opinion from my experiences with Tau.