As I couldn't find the answers to my own questions in any book - and I hope some of you knows a good title to be searched for information on this - I searched the web and found one quite extensive page on Roman religious system, priests and priesthoods: http://www.societasviaromana.org/Collegium_Religionis/priesthoods.htm

The following caption is from there, but I think we'd better check these facts before using them. I have made some clarifications [in brackets].

"The collegium [Pontificum] was founded by Numa [Numa Pompilius 2nd king of Rome] to assist the king in religious matters. Originally all three pontifices were Patricians, but it was then opened to Plebeians by the Lex Ogulnia of 300 BCE, raising its membership to nine (four Patricians and five Plebeians). By the late Republic it consisted of fifteen members (7 Patricians
and 8 Plebeians), including the Rex Sacrorum, the Pontifex Maximus and the Flamines Maiores - Flamen Dialis, Flamen Martialis and Flamen Quirinalis. Patricians traditionally held all of these latter positions. Then in 106 BCE the Pontifex Maximus was opened to Plebeians. Pontifical offices were initially filled by the Collegium itself, which also elected the Pontifex Maximus. By 250 BCE the Pontifex Maximus was elected in the Comitia Populi Tributa, and by 103 BCE the comitia elected the other pontifices from candidates put up by the Collegium."

This sounds quite reasonable, but tell me if you can find any texts that contradict this or argue differently. I will keep on searching as well - the sources listed on that page are a starting point, though I doubt I will find many of them here.