Yes, Buddha himself did fix the rules in his own words, and yes, Buddha was a monk.
The texts regarding the rules for monks and nuns is called the Vinaya Pitaka.
Here is a source where you can download pdf's of the actual different volumes of texts, both in the original Pali and English translations.
Here is an excerpt of an article by a Buddhist monk which explains the origin for the rules for Buddhist monks and nuns:
Introduction to the Patimokkha Rules
The Buddha did not set out a code of rules all at once. Instead, he formulated rules one by one, in response to particular incidents. The Canon reports these incidents in each case, and often a knowledge of these "origin stories" can help in understanding the reasons behind the rules. For instance, the origin story to the rule forbidding lustful conduct between monks and women shows that the Buddha did not view women as somehow inferior or unclean. Rather, the rule comes from an incident where a monk was fondling the wife of a Brahman who had come to visit his hut, and the Buddha wanted women to feel safe in the knowledge that when visiting monasteries they would not be in danger of being molested.
Some of the stories are classics of Buddhist literature, and show a dry, understated sense of humor together with a perceptive insight into human foibles. The element of humor here is very important, for without it there can be no intelligent set of rules to govern human behavior.
As time passed, and the number of rules grew, some of the Buddha's followers, headed by Ven. Upali, gathered the major rules into a set code the Patimokkha that eventually contained 227 rules. The minor rules, which came to number several hundred, they gathered into chapters loosely organized according to topic, called Khandhakas.
The Patimokkha as we now have it is embedded in a text called the Sutta Vibhanga. This presents each rule, preceded by its origin story, and followed by what permutations, if any, it went through before reaching its final form. The rule is then analyzed into its component elements, to show how the factors of effort, object, perception, intention and result do or do not mitigate the penalty assigned by the rule. The discussion then concludes with a list of extenuating circumstances for which there is no offense in breaking the rule.
It is interesting to note that while there is a wide variety of traditions practiced within Buddhism, the rules for monks and nuns are all very similar across all of the various Buddhist traditions.
