I have a Space Marine Fleet.
Battle Barge
4 Strike Cruisers
2 Squadrons of 4 Novas
Tau are much more of an Ordnance fleet, since they can put out a huge number of guided missiles, which are deadly. You don't want to get up close and slug it out if you're Tau. You will be crushed.
Avoid boarding actions. Avoid being rammed. Avoid being shot. Tau are really quite squishy, but they can put out so many Mantas and missiles that it shouldn't be much of a problem. Also, download the newest FAQ and errata, as it is definitely a better game with those revisions.
Tactics wise, don't get split. Tau are better in groups. As an example, if I were playing you I would screen ahead with my Nova squadrons, and then dive my cruisers and battle barge straight for your center. If they got in among your ships then it would be pretty much over. Space Marines are a hammer. Use your speed to avoid, but don't let your escorts run off chasing other escorts. Control the movement of the game, and you will do well with Tau. Don't let Orcs or 'Nids get anywhere near you.