1) - As far as my understanding of Mercenaries goes, they usually were proffessional soldiers. They tended to travel so they could serve as soldiers for foreign armies. In most cases they would serve most armies, though there are exceptions. While a Galatian would serve Pontus, the ptolemaioi, or the seleucids without problem, certain mercenaries would only serve certain nations. For example a gaesat wouldn't serve a Roman commander. Certain areas were also known for gathering of mercenaries looking for jobs. Remember that place in in the Pelleponesos which has those 4 different province borders in it? That place was a famous spot where mercenaries from a whole lot of places came together. Certain peoples where used as mercenaries in really a lot of places and in vast numbers. For example the Kretan Archers, numidian archers, Rhodian slingers, Galatians. Bigger good organized nations tended to use them at the complete different side of their empire, so that they would help out the natives or start rebellions on their own. Romans for example would use Brittons in germany, and germans in Britanny.
- Yes It happened. Numidians have been used by the Cartagenians and later the romans as mercenaries. The Romans especially liked their archers, but their light cavalry where also used much by both sides during the punic wars.
Gauls just served as mercenaries for the highest paycheck. Though Gaesats, one of the more famous mercenaries, apparantly weren't prepared to serve romans. Germans served as mercenaries a lot, especially from Caesar onwards. They would make out the biggest part of the roman army by the time it started to crumble.
- Usually it goes the other way around. When a nation gets stronger and richer it tends to use mercenaries more. The best example of this are the Carthagenians. You could also say that the Romans started to use more mercenaries as they grew in time. Though most foreigners (Germans), in Roman armies would serve the same way as the other legionaries.