Well, the answers to your questions is, of course, it depends!
Troops with shields fare a lot better against arrows, some fortresses have no arrow towers, some have a bunch. Is your enemy using archers or mages, if so how many, how powerful? Do you have stronger ranged troops than your enemy? Do you have any resistances you can throw down, like for instance fire resistance if you're expecting fire mages pummeling you? Do you have a small number of tough guys who you can buff, or a huge army of chaff? Do you expect them to send a never ending wave of undead out the front gate? Do you/they have cavalry or flyers? Etc, etc, etc. This game has very deep strategy, you can't really give a one size fits all answer to questions like that.
As far as fatigue goes, you'll start getting a handle on that when you get more powerful in magic and start summoning big bad dudes who are basically invincible...until they pass out from exhaustion.
It's not usually a big deal with your swarms of regular units. Just one more dimension you have to ballance.