I'll be unable to answer during the day because of work schedule but will get back as soon as I can.

There is a strange agency dispatch that alledges that the election date could be postponed after the High Court (Consiglio di Stato) accepted to discuss a case. If the Court were to decide in favour of a party that was excluded from participating over the usage of the old DC cross as a symbol, the elections would be postponed to thirty days after the decision. A disaster!

1) The Rainbow Party consists of the Communist Refoundation Party (Fausto Bertinotti), the Democratic Left (Fabio Mussi and Gavino Angius), the Party of Italian Communists (Oliviero Diliberto) and the Federation of the Greens (Pecoraro Scanio). The group consolidated itself in the wake of Veltroni's refusal to run with other parties or coalitions. There are several micro-parties on the far left that broke away from Refoundation.
2)The Udeur fell apart after Mastella withdrew his support for Prodi. Most of the party members refused to follow suit and leave the centerleft coalition both on a national level and regional level. It was clear that had Mastella passed on over to Berlusconi, the party would have kicked him out. The Udeur is not participating in the elections.
Another party, Boselli's Socialist Party, is running alone after Veltroni refused to take them on board.

3) It becomes a constitutional dilemma since in theory the "prime minister" is elected as head of a coalition. So an alliance of three groups against the coalition that expresses the "prime minister" would probably end up in the courts- and the streets.

Veltroni has said he has no intention of forming a government with anyone. Berlusconi of course will have no problem wooing back Casini.