Those examples, are dependent on the deck type youre building: Aggressive : You want more creatures hitting the field and chipping at your opponents life points and/or defending yours until your win condition is met
For example, even if we interpreted your library as part of the battlefield for this purpose, slipping one into the card sleeve of a card in your library during a search effectively marks your deck, and thats not allowed
He may be Goofy, but he didn't come to mess around
Instead of having lots of hoops to jump through to make Angels, all you need to do here is put an artifact into play
I think most blue artifact decks can find some space for this one
Captain Eberhart is a two-mana 1/1 legendary Human Soldier with double strike that makes the cards you drew this turn one mana cheaper while making opponents cards they drew this turn one mana more expensive