Can we sacrifice stuff for personal abilities? Like gaining more strength or mental skill, instead of summoning things?
No, because that would defeat the point of being an Overlord. Take, for example, Dr. Doom. If he were an Overlord, he'd be a Psychic-ability Overlord. He has his own set of skills. He doesn't acquire more, or others, but he has a keen mind and does his job in his own way. Likewise, you are a Steampunk-Technology Oriented Overlord. You have your own set of skills, which may compliment a specific fighting style rather than another. Of course being an Overlord, you have your very own powers.
For example, you aren't actually giving Gatlings to Minions. You are getting Upgraded Materials to Upgrade minions. That the upgraded minion wields a Gatling Gun or a Scythe, if he has ten arms or twenty feet is inconsequential. You are free to customize the troops as much as you like, but the bonuses are fixed but vary only on their ways of achieving success. So, scythes gives a +1! Bladed Edges! when attacking and Reinforced Carapace gives a +1! Defensive Stuff! when suffering attacks.
And the purpose of having minions would be null and void, if sacrificing them gave you their ability in a way or another.
Mooks are there to be meat shields and fall by the dozens against the heroes. Dark Minions are there to be tougher but not by much. Dark Advisors are like the Sergeants of Darkness' Army, and you are the Overlord, the Final Boss.
And your enemies can be the Sailor Moon, the DragonBall, the 'Insert Hero type' here...but in the end, you don't acquire more skills...you acquire more people with different skills.
You're a Pokémon Catcher of Evil Creatures. Which makes it all the more interesting. Shall I send Succubus on the kidnapping mission, knowing they don't have Destruction bonuses? Shall I send Only Dark Minions, knowing they will sap more damage before falling, but also that replacing one each will cost me a turn? Shall I send Chemista, or Psysquid? Shall I send Gamington and a horde of Minions for Zerg Rush? There is a reason 'Choosing the Team' is a step. Because it's
important.
Dark Advisors are like the Anti-Hero. But they stand on an even ground. You are the Overlord. You are one step above the Heroes, but you, yourself, are on Even Ground with a
group of heroes. And a Heroic Hero is a
little step above Heroes. The Heroic Hero is like Vegeta, or like Grey, or like Professor Oak. He's the Sasuke or the Hinata or the Sakura. He's not the 'Main Protagonist' but he's just a little bit behind him. There aren't many of them, but they exist. They are
not to be taken lightly.
Furthermore, I indeed told you that 'having more than one Dark Advisor' is inadvisable at the beginning, and then made it a rule of only having one per mission...(somewhere further down, I'm sure) BUT! I never said anything about there not being also the Overlord.
So yes, You could make a Overlord+(Chemista+Cerulean)-Count as one+ 8 creatures. You could assign Cerulean as your side-kick if you so wish.
But monsters can only be used for destruction missions, not Mixed ones.
You can try and Burn Up the Cash/Money on the Steel Piggybank. Who knows what will happen...
Ps: The Overlord is Pure Control. He has a -1 to Destruction related stuff, but a +2 on Control related stuff. at least, that's the basic thing you know of.
On Postponing missions: Nope. Missions might 'reappear' along the way once more, but they can't be postponed. It's the Choice in the end that makes the Overlord. Taking a mission rather than another unlocks something rather than something else. You have to make choices.
And choices have consequences.Ps: Catfishes are static. They cannot leave the Lair's proximity. And they're aquatic. And Destruction Only.