Good onya mate
Well with the 4 week campaign and 1 day tournament coming up in June, it's a good time to get into Epic!
I really should get some threads started soon...
I suggest Matt is the local authority on Tau at the moment. He has designed his own list which makes use of some of the newer Tau toys and hopes to help them play a slightly different game than 'try and shoot everything off the table before it clobbers you...'
I heard that there was possibly a local Tau army up for sale - maybe contact Reaper to see if he still has them (heaps of Forgeworld goodies in it).
I love the Onslaught Miniatures stuff as well and plan to use them in the future.
Fire Warriors are usually my preferred Break Their Spirit formation (most expensive formation in the army). You could bulk out a foot slogger
formation with some goodies (1 Skyray, Pathfinders, extra Firewarriors, Ethereal etc) to act as a Blitz Guard (protect your Blitz objective). This has the advantage of starting the game Garrisoned on Overwatch and can put out a nasty amount of anti personnel fire power.
Usually, Epic is a game that's won by being in the right place at the right time. This means mobility. Firewarriors in Devilfish is a great idea to supply that mobility.
Crisis Suits are fantastic but somewhat susceptible to engagements (as most of the Tau are) due to small formation size and no reinforced armour (mind you, 3+ armour save is great against normal attacks and a 6+ invulnerable save is helpful, even against Titan Killing hits). Another great use for Crisis Suits is as a payload for an Orca transport. It's a very shooty formation that can be deleivered by air to any part of the board (just make sure you nullify enemy AA first as Orcas are made of tissue paper
Pathfinders are one of those formation that your opponent can't decide what to do with. They are not a very threatening formation by themselves but once you take into consideration their markerlights, they a vital to the rest of the Tau army working well. This can also be accomplished with Tetras at a cheaper price though.
Hammerheads are very good at killing lightly armoured enemy vehicles. They struggle a bit once placed up against reinforced armour though. So much so that there was a recent push to get their weapon stats changed, to no avail so far
. They are fast, shooty and manoeuvrable, just don;t expevt them to be able to go toe to toe with Leman Russ tanks!
Barracudas are pretty good as AA units. They're less good at hitting things on the ground though. Good for placing blast markers and going after isolated, broken formations. Skyrays are excellent AA and should be included as upgrades wherever possible. Upgrade Skyrays are better that the dedicated Skyray formation which is easily broken/nullified.
Tigersharks are ok. The AX-1-0 is the Tau main access to titan killing weapons (often referred to as a flying Shadowsword). They are a bit expensive and will every now and then fail to activate which is REALLY annoying. Taking the space ship is another good/cheap source of titan killing power.
Hopefully, that covers most of your questions (from the PM as well).
Sing out for more info and Matt will be your best friend if you play his list!