Warhammer 40K Tactics

[Wraith]

Hmm.. I am planning to do a Gue'Vera Guard unit, so fluff-wise, I wouldn't accept DaemonHunter's "reinforcement", I would have to go with Tau reinforcement. Do the Tau have good HtH units to stand against Space Marines? (I know the Tau are even more fragile than the IG in HtH, but maybe they got auxiliaries?)

Tau really aren't built for Close Combat - it isn't a rumour or a generalisation, they are quite distinctly biased against it.

All Tau are WS2, with the exception of some of their "Squad Sergeant" models which are WS3, Stealth Suits (WS3) and anything wearing a VX8 Crisis Suit (WS3). Only 2 Tau models in the entire army have power weapons, and both are special characters.
If you really, really want to use Tau in CC, Crisis suits are probably your only hope as - although they are Elites and therefore very expensive to take risks with - they do have A2 as standard, and their Strength/Armour Save will serve them better than most things.

By and large, Kroot are your best bet for close combat. They're not much better fighters than Tau, but they have more attacks and are a lot cheaper, which means they're perfect for tying up an enemy while you surround them with your Tanks and missile launchers rather than sacrifice some of your precious guns to do so.

Against Marines, I'd recommend that you stay out of Close Combat at all costs.
Failing that, either take a LOT of Kroot, try to justify the inclusion of Ogryns into your army, or instead invest in Commander Farsight and some of his Bodyguards. The latter is probably the best you can hope for, short of some really, REALLY lucky rolls with your flechette launchers...

Hum.. how good are sniper rifles against Space Marines, Necrons, and other high-toughness enemies?

Variable, depending on what kind of Snipers you have. Human and Tau Snipers have slightly different rules to Eldar, and I only play the latter, but here's what I have learned.
They're just too expensive to use en masse, making them only annoyances against horde armies, and their AP just isn't up to the task of reliably taking out anything in Carapace (4+) Armour or better. Unless you're assured of seeing lots of Independent Characters in your opponents' armies, which are sometimes a viable target depending on who they are and what they're doing, I wouldn't bother.

Having said that, they're sometimes quite useful for taking out Large, Tough Creatures like Wraithlord and Carnifex. These sorts of things usually require anti-tank weaponry to take down, which can often be put to better use taking out tanks and larger numbers of closely packed infantry.
Snipers ignore ridiculously high toughness values, and Monstrous Creatures generally lack a brilliant armour save, so a Squad of 5 or 6 Snipers acting without help from other units can usually be trusted to take out something like a Wraithlord in a couple of turns or so.

So against Marines and Necrons? The answer is usually "Not much, because their armour is more use to them than their Toughness and Sniper Rifles prefer it the other way around".

Does anyone have Imperial Armour 1, and if so, would you be willing to look up the stats of the Salamander to see if it has any transport space?

No, they don't. Your original choice in the Centaur is a much better one