Consider this if I may throw this out there. A guardian vs a Fire Warrior is a comparison as I don't feel they measure up correctly. I, also, believe the Guardian is a bit overpointed by about 2 points actually (especially in comparison to the DE codex and their Warrior). So, I like to compare a Fire Warrior to a Veteran Guardsman as they are the same point value. The Veteran has better BS, WS, I, and I believe they even have better Leadership but their base gun is weaker than the Pulse Rifle. I look at it this way, is the extra 2 points of Str and 6" of range really worth the difference in base stat line? I don't actually believe so but that is my opinion. Why you see a lot of people running min. squads of Fire Warriors is so they do not have to pay the slightly inflated cost of a Fire Warrior when they could be spending those points on another XV8. For every 6 man, you can get 1 more really effective XV8 instead. Now, let's go a little deeper into this. The other Troop option you have is a Kroot Carnivore which is just about 1/3 cheaper than the Fire Warrior. What you get is a model with the same BS, better WS, better I, worse save when not in good cover, worse Leadership (I think), Infiltrate, and a little worse of a gun. You can mass up Kroot in Cover with their infiltration skill (especially if an objective is there) and now they have the same survivability. Kroot tend to me the big unit of the Tau Army.
Now, I am not saying to minimize your Fire Warrior contribution as I do max their squad whenever I can. I would say that, for a stand and shoot, Kroot are better and have the CC ability that a front line shooter should have. Properly employed, the two units should work in tandem well. Take a full Fire Warrior Squad in a Warfish and a 15 man Kroot Squad. Use them two to take down one unit, assaulting with the Kroot if you need a blockade for the Fire Warriors and you have a brilliant tactical combination. In the numbers though, you are talking about 2 units that, combined, cost over 300 points of your army total. In the end of it, you could have spent those 300 points on nearly 2 full units of XV8 that can put out more reliable, powerful, and functional fire power. Hence why you see minimized Fire Warrior Squads.
See the 2k list I put up a little bit ago today to see what I like to run. I have found it to be fairly effective against most players with exception to a Land Raider heavy, Monster Bug, Dark Eldar rapid assault, or a Blood Angel Force (darn the Blood Angels, I can never win). That list may be what you are kind of looking for in a Tau Troop Heavy Force.