Italian LINE infantry wear uniforms of French cut, but in Italian colors. Pierre Perconte has the best reference set on the web (
http://www.histunif.com/italie/italie2/ital000.htm) but it is in French. Still a gold mine of information.
For the 1st-7th Line, Victrix 1808-1812 French are appropriate. Note that the Italians did not have Eagles at this time - they were originally supposed to have lions, but that didn't work out, so they got Eagles after the 1809 campaign. The Italian Lozenge-pattern flags were not issued until after the 1809 campaign, so they fought under their Republican standards. Napitalia's units reference (
http://napitalia.org.uk/eng/histind.shtml) is good for the 1809 LINE infantry, but the light infantry are for 1812.
Italian LIGHT infantry wear a uniform of French light infantry cut, but they wear a czapska instead of a shako. The czapska is also different from the one worn by Poles - it's rotated 45° compared to what the Poles wear. As far as I know, nobody makes an Italian Light Infantry for 1800-1809. I also do not know if anybody makes Czapska heads in 25mm/28mm either. You'll have to do a lot of conversion work here. I have several articles on the Light Infantry if you want.
The Dalmation Light Infantry wore a single breasted coatee, but the cut was otherwise in French fashion (except the elite companies only wore epaulettes). That would require a bit of work, but you might be able to do them best using 28mm Nassauers and then adding epaulettes as necessary. You also may need to do some head swaps to get the grenadiers right. Earlier in the period, they wore Corsehuts. That would be a lot of head swap work, but you can get the heads from Victrix Landwehr plastics.
The Italian Royal Guard uniforms were the exact same cut as their counterparts in the Imperial Guard. Use Guard Grenadiers for the Italian Grenadiers and the Grenadier Velites, and use Guard Chasseurs for the Chasseurs and Chasseur Velites. There were no young guard in 1809. Histofig has a series of plates on the Italian Royal Guard here:
http://empire.histofig.com/-Garde-Royale,211-.html
The Italian units campaigned pretty hard along with the rest of the Army of Italy in 1809. Since they weren't at Wagram (except for the Royal Guard), it's most likely they fought in campaign uniforms. You can do some in full dress (the Guard should be in full dress), but campaign dress is more likely. Stuff wore out, after all.