It's not a problem to use Hydralisk/Mutalisk on minions or mercs. If the target dies, the Hydralisk will just attack the next available target. If no target remains, it will go to the nearest lane. This means you can safely cast it on a minion wave and it will help you clear it (until it expires), doing its full damage in the process. You can also cast in when clearing merc camps for the same effect (of course it might die earlier because it's tanking hits). The guide should be...

I'm not sure what I'm supposed to make of this "guide". Are you trolling? "Never attribute to malice that which is adequately explained by stupidity.", well. Anyways, you're writing this for noobs, as I assume, since "pros don't need tier lists" (your words). Why recommend Illidan then, or put him in "tier 1"? He's terribly weak without proper team support, and just by looking at his win rates quickly confirms that (the higher you go on...