As with anything 7th ed, the answer is it depends. There are so many more variables to take into consideration when building a 7th ed army than there were in previous editions. That said, I'll try to answer as best I can (obviously, this advice is only valid for Battleforged armies - Unbound is open slather).
The Tau army is now following a similar route to the Eldar armies - they synergize well with each other and a myriad of rules and effects mean that the whole army often performs better than the individual parts would suggest, and the Fire Warriors certainly come into this - the options they now have thanks to Supporting Fire, Cadre Fireblades, Ethereals and Markerlights (not to mention other effects I'm no doubt forgetting) mean that even though they are the basic troops choice, they're able to be quite effective. And it depends a lot on the composition of the rest of the army.
Build-wise there are two main detachments presented in the Codex - the CAD and the Hunter Contingent.
The Combined Arms Detachment is the closest thing to the previous editions Force Organisation Charts. All Troops choices gain Objective Secured - meaning that they are the only units in your army to be able to control or contest objectives. So, if you are choosing a CAD for your army, consideration needs to be given to the composition of the rest of your army, and the objectives that you, as the player, want to use to determine victory (i.e. Missions, Objectives, straight up kill points etc.) Under the CAD you must take 1 HQ and 2 Troops, at this point you need to make the decision of how the rest of your army is styled (as above).
If you want to focus on kill points and suits (and ignore objectives) - keep the troops to a minimum: 2 5 man Fire Warrior squads (Strike Teams) that can camp out on a backfield objective or generally just stay out of harms way. This is the minimum troop tax and largely dependent on your play style and points limit of the game and comes in at 90 pts for the two min sized squads.
Otherwise, 10-12 is the optimal size. This gives you good offensive output (30" Range, Supporting Fire etc), and the ability to contest and hold objectives and soak some casualties before the unit disappears. 10 is probably slightly more efficient Morale wise as there is no difference between 10 and 12 when it comes to determining 25% casualties (they both need to loose 3 models). The addition of drones and other goodies can also boost their survivability, but at increased points cost.
2. Hunter Contingent
This is the detachment formation in the Codex, and the minimum troop tax in the core formation (the Hunter Cadre) is 3 units of any mix of Striker Teams, Breach Teams and Kroot. Again, the points of the game will determine how much you want to spend on the core troops, but a similar approach to the CAD can be taken. If points are scarce, the minimum cost for 3 x 5 man squads is 135.
However, when you take a Cadre Fireblade as part of this formation, his Volley Fire rule allows his attached unit to fire an extra shot. That means a 12 strong squad is throwing out 26 S5 shots at 30" range and 39 shots at close range (12 Fire Warriors plus the Fireblade). He also let's them Split Fire which can be useful.
Adding an Ethereal (as part of the Contingent Headquarters Formation) will also let all units within 12" fire an extra shot at half range as well. The above unit of 12 Fire Warriors and the Fireblade then start pumping out 52 S5 shots at anything under 15" - which is also outside Rapid Fire range of nearly every other unit in the game.
The Formation and Detachment bonuses are also useful. The Detachment bonus lets multiple units fire as a single unit on the same target. If 3 or more units do so, then you gain +1 BS (30+ BS4 Fire Warriors anyone?). The Formation Bonus lets units within 12" use Supporting Fire (instead of 6"), and let's units within 12" of your Commander and/or Fireblade Shoot then Run/Flat Out or Run/Flat Out then Shoot in the same phase - meaning your infantry units can be surprisingly mobile with little to no loss in damage output.
The most effective size for Fire Warriors (Strike Teams at any rate) still seems to be 10-12, without a Transport.
Shas'ui upgrade is optional, but I would recommend it in units that do not benefit from having a Fireblade or ethereal. It's a relatively modest points investment for the boosted Ld.
The devilfish is a large points investment for something that doesn't add a great deal to the unit output, and spending points on multiple transports greatly decreases your ability to better spend those points elsewhere.
Used as part of the Hunter Contingent detachment, 3 full size Strike Teams (with added Ethereal and Fireblade) can form a relatively solid yet still damaging firebase able to contest objectives and control board sections.
Strike Teams are still my preferred option: The 30" range still effectively gives them an extra round of shooting over most other troops, S5 means they can threaten light vehicles (AV10-11), and they can put out a decent amount of shots per turn.