Although many on the PIP side do argue that Rudy was a serial burglar, was a drifter, was aggressive with women, etc., I agree that the evidence for much of this is based on some conjecture, which works best if the person making the conjectures already believes that Rudy is guilty of Meredith's murder. But I think the reason that PIP posters often don't reply to challenges to this information is that it doesn't really matter.
I think that who Rudy was, and if he was a fence, or a burglar, or whatever, is interesting background, but largely immaterial. The evidence at the murder scene shows that Rudy Guede killed Meredith Kercher, by himself. Some of these other points support the idea that he was the kind of person that might end up in such a situation, and his behavior that night seems consistent with his previous behavior (in regards to breaking into offices, nursery schools, homes, etc.). But even if he was the Mayor of Perugia, and had never violated any laws before, he would still be guilty of the murder, and the evidence of that would be clear.
There may be some who can't get over the idea that Rudy had to have been a burglar, drug dealer, etc., but what I see when people argue that is that it makes sense, based on what evidence we have. But it is not essential to solving the crime. What is clear from the evidence is:
1) Mez was killed;
2) Rudy was there, and had never been there before;
3) He was there at the time the killing occured;
4) There is no reliable evidence that anyone else was there at that time;
5) His story about why he was there, and what happened when he was there, is not believable.
There may be a "dogma" around who Rudy was and why, but it is not a driver of the case, or the fact that Knox and Sollecito are innocent. Rudy did it. They didn't. That is true if Rudy was a burglar, or a fence, or a successful businessman. The story of Rudy as a drifter, burglar, and guy who annoyed women and fell asleep on toilets fits the narrative, and seems to be the most likely true story. But he is still the killer, regardless of how one wants to portray him.