how to deal with city number maintenance cost?

[AcaMetis]

Question about pyramids - i thought at higher difficulty levels, building wonders was impossible, so i shouldn't rely on them at lower difficulty levels?
Question about chopping - I was concerned that in flat areas, if i chopped all the trees, then there wouldn't be any source of hammers. is it good to leave a forest or two, or in general its better to chop down all the trees?

"Impossible", no, but there's a lot of wonders that you can afford to build on Noble that you wouldn't even want to bother trying to build on Deity. Well, build to complete on Deity. Build for failgold, absolutely, but on Deity you'd never build Stonehenge to build Stonehenge. Two Workers or a Settler plus Warrior is worth more than building SH. As are the beakers you'd invest in Mysticism, if you didn't start with it.

There's rare situation where you'd want to save forests, but general rule: Chop all the trees with the most extreme of arboreal prejudice. Hammers now is worth more than a decent-ish tile and half a several thousand years down the line, and you shouldn't endeavour to work unimproved tiles if you can help it anyway. If a city lacks production than use the whip to get stuff out, and if you ever need the city to not grow on it's abundant food tiles that other nearby cities can't take from it, run specialists.