Bartleby pretty well covered all of these, but I'd add just a couple of comments:
2. have recently moved from Warlord to Regent level and am NOT doing well. The other civs are miles ahead with technology, so I can't be a tech broker. When I offer to trade spices / other goodies, they offer ridiculously low payments (6 Gold for 1 spice !!!!!!!!!!!)
At Regent, the AI just won't have much gold for trade. However, as Bartleby noted, with proper management and a little practice, you should be out-researching the AI at this level. Some keys to that are: (1) lots of roads for the +1 commerce; and (2) making early contacts. Stack a few galleys and send them in search of the other continent. The AI doesn't use suicide galleys, so this can put you in the position of tech broker.
Also, the commerce bonus for Republic goes a long, long way towards both supporting troops and fueling research.
3. what do people recommend government wise (I usually like to attack other civs rather than try and win by scientific advance, or I find myself winning domination-wise because my civ gets so big):
Despotism-Monarchy and stay there?
Despotism-Republic- but what about war weariness?
How many changes of govmt do you guys usually "suffer"?
What version are you playing? If it's Conquests, Despotism-Republic-stay-there. (If it's not, I'll let someone who knows something about the governments in Vanilla and PTW answer your question). Anyway, combat war weariness by using minimizing losses and using oscillating wars. No need for a second switch. I've heard that religious civs might pull a second switch, but I rarely play religious civs and the thought of 7 turns of anarchy when I've got 175 cities is downright painful.
Also, run a search in the forums for governments. You'll find that this topic comes up quite regularly and that there have been some very enlightening discussions on it.
5. Are there "advantageous" civs to be warmongering (ie have you found that one HAS to be a militaristic civ, or would one be successful as e.g. scientific/industrious civ as well?)
You can successfully warmonger with a variety of civs and civ traits. In addition to Bartleby's comments on the Ottomans, Persians and Indians, take a look at the Iroquois. Agricultural and Commercial, but they're one of the most popular civs on this board.