Did Hitler inform Mussolini of his decision to attack Poland in advance? Was Mussolini in favour?
Mussolini learnt about the German intentions first as did most other countries, by diplomatic reports from his ambassador in Berlin and similar sources; alarmed, in early August 1939, Mussolini sent Galeazzo Ciano for a meeting with Ribbentrop, who told him Germany intended to invade the whole of Poland, not just Danzig. Mussolini was clearly against it, because he realized it would mean a war against the United Kingdom and France that Italy was not ready for, so he tried to organise an international conference like the one held at Munich the year before, but failed because Hitler was not interested in any such conference.
Check pages 248-250 from Mussolini by M. Clark for more info.
Excerpt of a letter from Hitler to Mussolini, dated August 25, 1939.
DUCE: (...) About the situation on the German-Polish frontier, I can only inform Your Excellency that we have been for weeks in a state of alarm, that as a result of the Polish mobilization German preparations have naturally also been increased, and that in case of an intolerable Polish action, I will act immediately.
The assertion of the Polish Government that it is not responsible for these inhuman proceedings, for the numerous border incidents (last night alone there were twenty-one Polish border violations), and for the firing on the German airplanes, which had already received orders to travel to East Prussia over the sea in order to avoid incidents, shows only that the Polish Government has its excitable soldiery no longer under control.
Since yesterday Danzig has been blockaded by Polish troops, a situation which is unendurable. Under these circumstances no one can say what the next hour may bring. I can only assure you there is a limit beyond which I will not be pushed under any circumstances. (...) ADOLF HITLER
Following the "Pact of Steel" concluded in May 1939, Germany and Italy consulted on all major European matters, so Mussolini knew about Germany's plans to invade Poland no later than August, 1939.
Italy's response was the so-called "Molybdenum List," a long list of war materials, headed by molybdenum, that Italy would require before joining Germany in a war.
Answering the second part of your question, it is quite probable that Mussolini was against war with Poland, although I don't know what his opinion about German-Polish war was.
During the war (until 1942, IIRC) Poland and Italy were not in a state of war. This is of course officially, as Polish soldiers fought against Italians eg. in Africa (but as a part of British forces) and some Polish warships fought against Italian ones (however some commanders have objections here). The war was declared by Poland (maybe under British pressure), not Italy, so it might be assumed, that Mussolini was not interested in a war against Poland.
Also as an addendum to previous answers:
In the morning [of 25 Aug 1939], Adolf Hitler sent a message to Benito Mussolini, noting that the reason why Italy was not informed of the Molotov-Ribbentrop Pact was because Hitler had not imagined the negotiations would conclude so quickly. He also revealed to him that war was to commence soon, but failed to let him know that the planned invasion date was on the following day. Later on the same day, however, Hitler hesitated in the face of the Anglo-Polish mutual defense agreement; he would quickly decide to postpone the invasion date. (...) [source]