<I realize i'm running short on days in Paris (only 3 days/2 nights). Based on this how where could I reduce days?>
Nowhere. Skip Paris, you just don't have time for it on this trip. Just a hassle, not a joy if you go there.

<i was thinking about reducing it two 2 nights (3 days in venice, rather than 3 nights (4 days) is there really so much to keep occupied? i keep reading conflicting information (i.e: 1 day is enough, venice is too small and boring etc) so this confuses me>
Well, you keep reading conflicting information on whichever subject because there are just enough idiots out there that someone has positive or negative opinions on virtually everything. And just as there are people who still admire Stalin, there are people who think one day is just enough for Venice. Like always, it's crucial to determine which information is reliable... and if you read just a bit of serious information on art and architecture in Venice, you won't have any doubt whether there's enough to keep you occupied for 4 days. Four days! Of course, that's definitely not enough to scratch the surface of Venice; but it's ok for a first start (after which you'll have to come back again, and again, and again, and again - as soon as you've spent, in total, three or four months in Venice, you may say you know it, basically at least).

Ah yes, and by the way, the advice of doing Pisa as a daytrip from Florence is really bad. Pisa is such a nice and lively and young town (a large university!) that it's real fun to spend an evening there, particularly for young people.

I'll get shouted down for this ....

I'd skip Paris on this trip and do it on another occasion.

It is the furthest North, with the shortest days and the coldest weather.

Winter in Paris can be bitter and temperatures can go down well below zero with a cutting wind.

Peter

Not from me! I wouldn't be again cutting Paris. Or Rome. Or even the two cities in Spain. To make for more time in the places you do decide to go. Deleting ones that are furtherst away geographically from the others.

3. I assume we can use trains within Italy to move from venice-rome-florence, but haven't been able to find this info.

Well there are very frequent trains between those Italian cities and you will have NO problem getting on them- you can just buy all your tickets upon arrival in Venice. www.trenitalia.com has schedules and fares and online discounts that American credit cards just do not seem to work so you, if an American, wait until Italy and pay full fare. To learn about Italian trains peruse these excellent web sites: www.seat61.com; www.budgeteuropetravel.com; www.ricksteves.com.

well Paris is a major detour in the OP's proposed itinerary but IMO Paris is so so exceptional that three days there would be a real highlight - the best perhaps along with Venice, to me at least.

and if you are the type to hop overnight trains you can take one between Barcelona and Paris and Paris and Venice - saving hotel costs and time. These are hotel trains with comfy private doubles - on the Spanish one some private rooms even have WC/douche in the room.

with the aspect of saving hotel costs flying between bases may not be cheaper than Eurail. A railpass pays the basic train fare but you have to pay a supplement for the type of sleeping accommodation you wish - from a multi-person compartment to higher for a private compartment - but still those sleeping accommodations charges would be less than what i hear the average Fodorite pays for a hotel room - far less.