Hello, I'll try and answer your questions.
1) It's models for Units within 3", you must have mis-read the rule. It's more forgiving than you think.
2) It can be a bit resilient, but you'll have no ability to deal with enemy armour, or tougher troops. Also, Necron Warriors are not the greatest in melee, and can be dragged down that way. You'll need support for the warriors.
3) The size of the model is irrelevant. If a character has less than 10 wounds, it can't be targeted unless it's the closest target, or the attacking unit has a rule which ignores this restriction (like snipers). It's just the rule for characters, a bit silly, but very beneficial for those units!
4)yes and no. Models don't need the fly keyword to charge the monolith (it has to rules preventing non-fly units from engaging it). But, Units with the fly keyword (as per the 40k rules), can fall back and shoot. However, it is compleatly rubbish in melee.
I hope this helps.