I have a Crusader team in HC which I don't play a lot since I always risk a sudden "
connection lost to the server" and then all my toons are dead. This hasn't been happening since several weeks tho. They're like Paragon 50 ATM.
Of course you want to take as many defensive skills as you can, specially All Resistance. With 900-1.000 unbuffed All Resistance you'll find out that elites can't hurt you much even in Torment I, of course I assume that you won't be sitting ducks on 10 stacks of Molten effect...
The real problem is when you are feared or knockback'd, meaning when your team is split up. But even so, with high resists and if you don't panic, you should be ok.
Crusaders are the best choice for multi-boxing HC because they are the toughest class. Eventually you can leave one spot for a Barbarian or a Monk in order to have more buffs. Including a Monk is probably the more efficient, however he'll be weaker that the Crusaders, even if he wears a shield. Which means he might die if you don't focus, since your Crusaders will be sustaining damage, but suddenly you'll figure out that your Monk is dead
About tactics, it's the usual multiboxing stuff, try to always be close to a wall or edge of a map so that if the team is split you can always regroup it by having the toons hit a corner or an angle, whatever.
If you have a Wizard on your main account, I suggest you use it to level your toons. Have one of your level 1 Crusaders begin a bounty or the main story in Torment VI (I recommend doing "Rumford at the Gate" in act I again and again), then invite your others level 1 Crusaders, and then your level 70 Wizard. Kill everything from the distance with your Wizard, and you'll be level 30 in no time. Be careful of monsters with arrows or spikes etc. they'll one-shot your Crusaders.
Every 10 levels or so, craft some armor for your Crusaders with a lot of Vitality on every piece. Materials aren't bound to account until level 61 so...
At level 30-35 Torment VI you'll find out that you can't kill stuff as fast with your Wizard, so you will have to gradually drop Torment levels. Eventually from there you can do the whole story in Normal mode... Be sure that you always invite your level 70 with one lower-level character.
Oh, also, this is very stupid, but when the Crusaders hit level 2 or 3, they will be granted (on your mouse right-clik button) a skill that makes them rush to the target, which can get them killed if you don't pay attention
Good luck