Reading about Junio Valerio Borghese, a naval officer in the Regia Marina, it notes that he was a "prominent hard-line fascist politician in post-war Italy". Given how "prominent hard-line Nazi politician in post-war Germany" would be pretty much impossible, I'm assuming that there was some sort of difference, but I'm not finding where exactly the line was.
Wikipedia says about the Italian Social Movement (MSI - post-war Fascist party) "however, due to the post-war Italian constitution and agreements with the Allied forces, advocating a return to fascism had to be done discreetly," so clearly some sort of barrier was created, but there mere fact the party was allowed to exist suggests nothing approaching the situation in post-war Germany was implemented.
So, a few questions:
What was the immediate post-war policy by the Allies in regards to the Italian Fascist parties and their members?
What sort of "Defascistization" process, if any, would party members have to go through?
Were members only of the National Fascist Party treated categorically different than those who had joined the Republican Fascist Party in 1943?
What sort of institutional barriers were created in Italy to deal with former party members who were unrepentant, and/or continued to support parties with a Fascist ideology?
What sort of platform was the MSI advocating just after the end of the war when it was founded in 1946? What did the Allies have to say about their platform and their involvement in Italian politics?
The allies policy to Italy was designed to keep them firmly in a NATO/ anti soviet camp. With that in mind the allies tried to be as light on Italy as possible. The allies for example did not have any sort of official tribunal for Italian war criminals and there was no allied authority in Italy, so it was up to the Italian government to prosecute the war criminals. There were a few independent tribunals for the really heinous war criminals, but these were no where as harsh as they should have been and the criminals were generally given a slap on the wrist. I like to bring up the example of Rodolfo Graziani who committed horrible crimes in Libya and Ethiopia and yet ende up serving only a pitifully small amount of time in prison.
It should also be remembered that Allied policy towards Italy was influenced by Italy's decision to switch sides halfway through and the new Italian government that was formed by marshal Bagdalio was considered legitimate. And many of the orginal fascists who supported Mussolini had went over to the new government, which made prosecuting them a bit more difficult.
There was in no way a "de-nazification" process in Italy, at least not in anyway comparable to the one that occurred in Germany. Italian war crimes unfortunately were overlooked by the allies in favour of prosecuting Germany. The war crimes and reprisals against civilians that occurred in Greece and the war crimes committed in the Balkans were pretty much entirely ignored by the allies and were basically swept under the rug by the Italians, who wanted to harness their reputation on the international stage. Probably the biggest change that was made in post war Italy was te removal of the monarchy which had once been popular but their suppor of Mussolini and the disastrous war had soured the public's opinion of the monarchy. The removal of the monarchy, can be seen as probably a strike at the far right, who the monarchy had been a supporter of.
Post-war Italy was also in a complete state of chaos. Peasants were starving, and the government exercised very little control, and roving bands of peasants, former guerrillas, and armed thugs hired by land owners roamed the land dealing out their forms of justice. Purges of former fascists were common, especially in the North where officials who had served the fascist government were either arrested, or killed. However, the Badoglio government put a stop to the indiscriminate killings as soon they regained control over the country. In a country like Italy which had a very noticeable North/South split, unity was seen as key in rebuilding the nation.
The National Liberation Committee, or the CLN who held control over the North, before Bagdalio's government gained control, held their own little "trials" for the most heinous fascists. Members of the fascist militia, and former black shirts were "tried" and faced harsh penalties. So in the end the Partisans were harsher on war criminals and former fascists the Badoglio government was. Now large amounts of fascists did receive temporary prison sentences, in 1946 there were around 40,000 ex fascists in prison; but this can be both viewed as a punishment and a way to protect them from reprisals. Not that it did much, partisans were notorious for breaking into jails and massacring the fascist prison population. Given the huge amount of violence and stark prison divides, it seems pretty clear why the Italian government took a reconciliation approach as opposed to the approach taken in post war Germany.
To answer some of your questions more directly:
Were members only of the National Fascist Party treated categorically different than those who had joined the Republican Fascist Party in 1943
Yes, if only because the majority of fascists who had joined before 1943 had a few years to distance themselves from Mussolini and the fascists, those who joined after 1943 had no such luck.
What sort of platform was the MSI advocating just after the end of the war when it was founded in 1946?
The immediate and long term goal of the MSI is to rehabilitate the fascist movement and more importantly, to portray Italy as a victim in World War 2. They were also anti-communist to an extreme. Which given the prevalence of communism in the new Italy made for a very divided Italian republic. In the late 40's they began to develop a number of social and welfare programs to support former fascists and protect them from reprisals. They were seen as a useful countermeasure to the communists, the allies wanted to prevent any sort of social revolution in Italy and the far-right was a useful tool.
Some good books on the matter are:
Mussolini, Italy, Italians, and the Second World War by Phillip Morgan
Mussolini by Dennis Mack smith
Fascist Ideology, Territory, and Expansion by Aristotle Kallis
Fascist Italy and Nazi Germany by Alexander De Grand