Does he still have that right now? Will he show up in court to say he choses silence in face of each question of the defense?
No, Guede can be forced to testify, in the same way as the witnesses today were forced to testify. If they invoke their right to silence, they can be found in contempt and given additional jail time. It's only when one is a defendant that one can invoke the right not to testify in one's own defence. I suspect that Guede would retain the right against self-incrimination, even when testifying as a witness, but that's a different matter.
Incidentally, I think that most people (especially the pro-guilt mob, who have a vested interest in getting it wrong) are misunderstanding the mechanism whereby the five inmates came to be testifying today. Sollecito's appeal asked that they be heard so that their stories could be tested in open court. The defence teams didn't argue that the inmates' stories were necessarily believable - only that they needed to be heard and tested in order to properly determine their veracity (or lack of). This is a vestige of Italy's inquisitorial system - it's incumbent upon the courts to use every reasonable means to determine the truth. In this case, the defence teams were essentially arguing that Massei's court had arbitrarily decided that the inmates' stories would add nothing to the court's search for the truth - the defences argued that this was an incorrect ruling, and that the inmates needed to be heard in order to make a proper determination.
Therefore, when I read people saying things like "This could backfire hugely on the defence if the inmates are shown to be telling a pack of lies", that's completely incorrect - and probably willfully so. The defence only ever argued that these inmates should be heard in open court, and that all parties should have the opportunity to question them and test their stories. Of course, if either Alessi or Aviello can be determined to be telling the truth, then this would be positive for Knox's/Sollecito's defence. But if they are all revealed to be liars, fantasists or simply utterly unreliable, then it's not a blow at all to the defence.
Think of a counter-example as an analogy. Suppose that a man had contacted the court to say that he had seen Knox and Sollecito coming out of the front door of the girls' cottage at 11.45pm with blood all over their hands and both brandishing knives. Suppose that the court decided that this "witness" should testify and be cross-examined by all parties. If the witness was examined in court and his testimony were found to be reliable and accurate, then this would be a huge development in favour of the prosecution, and against Knox's/Sollecito's defence.
But suppose instead that this "witness" was discovered during the court examination to be a serial liar in important criminal cases, and that the man was actually verifiably somewhere else in Perugia at the time when he had claimed to have witnessed Knox and Sollecito exiting the cottage. His testimony would therefore be ruled to be unreliable and worthless. But this would not count "against" the prosecution, or "for" Knox/Sollecito. It would merely not move the case one way or another. And this is exactly the situation with the inmate testimony: it will either be positive for Knox/Sollecito, or it will be neutral. It cannot harm Knox/Sollecito if the inmates are found to be unreliable or inaccurate.
Lastly, it was virtually inevitable that Guede would be called to the stand once the court decided that there might be some merit to Alessi's (and the three other corroborating inmates') story. This is because the testimony of Alessi and the other three is no more than hearsay as it affects the case against Knox and Sollecito. The court will need to hear primary testimony from Guede before it makes a proper decision as to whether it's likely that he indeed told Alessi that neither Knox nor Sollecito were involved.
Lastly lastly(!), I know that Twitter is only a frivolous endeavour, but Latza Nadeau has really shown her true colours in her court commentary today. The testimony of Alessi, Aviello and the other inmates should be judged on its merits, with the obvious caveat that they are all serving prison sentences for crimes of varying severity. But to refer to Alessi throughout as "child killer" or "baby killer" (and to Aviello as "mafioso brother") is ludicrously inflammatory and unnecessary. She also doesn't seem to get the reason why their testimony is being heard, which makes a further mockery of her credentials as a so-called "serious journalist". She's a joke, I'm afraid.