do more (unnecessary) buildings and units add to the maintanance of your civ?
Definitely.
- Don't build any health buildings in cities that are nowhere near unhealthiness (exception: an aqueduct in a healthy production city if you want to go for the Hanging Gardens).
- No early barracks in cities that won't produce units, though I tend to eventually build barracks everywhere at later eras.
- Unless maintenance is very low (like for Charlemagne), courthouses are important - even in the capital now for the espionage bonus
- No science buildings in cities that mostly produce hammers unless you need them to be able to build Oxford.
- No religious buildings in production cities that are happy unless I need them for cathedrals or culture
- No financial buildings in cities that produce mostly hammers and little gold. A bank in particular is very expensive - you want that 50% bonus to be worth the hammers spent on the bank.
- Forges I initially limit to cities where they're worth the extra hammers and won't force me to build health buildings for nothing, but I systematically build them if I have 8-12 cities, with one or two extras in case I lose cities in war. Same for banks - I'll build 8 in my best cities gold-wise to get WS, and same for Universities.
- Even if you play a peaceful style, you need to keep military units a priority over pretty much everything, including wonders. You can live without wonders, but wonders help little if you get attacked and have spent years building wonders instead of troops.
To play peacefully and don't get smashed, you pretty much need to keep a good lead in tech and a very strong defensive army and walls, castles and the likes (offensive units are less important for this style, I find, though in warlords I liked to keep some as deterrents and to fight back a bit, at least.).
-which city would great library and/or oxford go?
Cities with a lot of science (cottage farms and some coastal cities, usually - especially when I play as the Dutch), or for the GL it can also go in the GP farm for the two free scientists. My rule of thumb for Oxford (and the first Academy) is to place them in the city that produces the most
(same for other academies), unless I plan to need the slot for another national wonder there (I often want Wall Street at the same place I could put Oxford). For Great Scientists, it's also necessary to calculate if an extra Academy is worth it, and if the GS wouldn't be more worthwile settled as super specialist in your science city where its effect will be multiplied.
Many wonders are useless or close to depending on your game style and which civ you're playing.
I'm not a terrific player overall (I'm stuck at Noble on BTS, used to play Prince for a challenge and Noble for games in the comfort zone) but I tend to go for only a few key early wonders unless I'm so much in advance I can afford to have one or two "Wonder cities". Which ones very much depend on your civ/leader, your style and your strategy. As the Dutch and following a peaceful strategy (I focus on Diplomatic, Cultural or Space Race/Time victories) I favour the Great Lighthouse and the Oracle first and pick Code of Laws as free tech, then use the Great Prophet to get Theology and build the AP. If the circumstances are favorable, I will build the Great Wall (since BTS), and for defense (shorter wars against you, or more costly) I find the Statue of Zeus interesting. For later Wonders I pick what I need depending on how my game is going, and occasionally what I can't afford to let some rivals have (I don't want to let a civ nearing cultural victory get Broadway/Hollywood/R&R and the Eiffel Tower, for instance). Typically, I have a few early key wonders, a few ones mid-game and most of the late game ones.