REXing is done below the happy-cap. Raising happiness with either Monarchy or Calendar is phase two, and shouldn't occur until you've claimed (or blocked off) all the land you can.
Technically, REXing is defined by food and hammers, not commerce. You want to improve your best tiles (food resources first, then hammer resources, then mine some hills), which means workers. And bring them under cultivation, which means city growth. And stake out new sites, which means settlers. All while avoiding barbarians, which means fog-busters. So it's a balancing act. Don't bother with barracks or granary -- build fogbusters while growing (unless you can foresee that the fogbuster won't finish and will decay in the queue, but growth is still the priority; in which case banking the hammers in a barracks/granary might be necessary).
Roughly speaking, I think it's best to grow onto all of your food/hammer resources before spitting out a settler. The increased tile yield will be worth it, unless the neighbors are really close and key city sites must be claimed. In some cases it might be worth it to grow onto mines if you have a high food surplus, though not necessarily. If my capital has a high food surplus, it may spit out one settler at size 2 or 3, then grow to the happy cap while building fogbusters, then become a worker/settler factory, with one warrior for garrison duty. If it's not high food surplus, I may not grow it to it's happy-cap for a while (depending on the balance between the need for workers/settlers and the need for fog-busters), and the garrison isn't needed.
When settling, the issues are: Proximity, less walking time, less barb protection needed, lower maintenance; Early growth potential, especially resources that can be worked without a border pop; Strategic resources, mainly Copper but sometimes Horses or Marble/Stone if that's your gameplan; and Blocking -- on high levels, the AIs will expand faster than you, both faster than you physically could and faster than you could economically support. If you want to have near-parity in land, you need to block off some territory that you can settle at your leisure.
Chopping: Building a worker to chop is an investment of 60 food/hammers that returns 5 hammers/turn (normal speed). That's not as good as growing onto food/hammer resources (say, 24 food to add a +2F+2H grassland Cow as your second tile); it's about equivalent to growing onto a mine (say, 26 food to add a -1F+3H grassland hill mine as your third tile), but with the opportunity cost that you can't chop those forests in the future, or use them for health. Overall I feel that chopping should be a secondary use of your workers, after improving resources and mines, but at that point it becomes a good return. Imperialistic and Expansionist leaders can leverage it a bit more. And if you settle a city that needs a border pop to access its best tiles, by all means chop the monument.
Of course, it all depends on the particulars. And commerce can't really be neglected forever -- if I start with floodplains, I may research Pottery first to get cottages growing, which doesn't play into a true REXing strategy, but it does keep me solvent. And there are alternatives to REXing, such as early Wonders.
Perhaps generate a game, save the start position, and then play it several times up to ~1000 BC to compare different approaches.
peace,
lilnev