I wouldn't worry about airports, since more or less all the ones I've seen (from the smallest to the huge ones) are pretty similar: you get your bags, you go though customs ("nothing to declare" - if you haven't brought a lot of cash / weapons / drugs with you).
I agree that that the easiest way to get to Piazzale Roma is by a shuttle bus. You can find the desks directly in the airport near the exit.
You can pay with credit card here, but I wouldn't advice getting money from an ATM: of course if it makes you feel safe not having cash with you at all, this has a meaning, but as I always do when I travel, I bring a couple of hundreds Euro with me (in your case Dollars), and change them in the center of the city I'm visiting (preferring Money-Exchanges without commission and with a good change rate). For bigger expenses I do pay with credit card (the only one not widely accepted in Italy is the American Express).
Thre reason I do not use my country's card to get money from a ATM abroad is that normally you have very high commission costs, plus the change rate is fixed and normally not very good. So if you want to get 100€, you are most probably going to pay 100$ + 6$ commissions + around 5$ bad rate.
Another thing that I normally avoid is changing money directly in the airport: their change rates are the worst and they normally do take commission.
About Venice: it is a quite safe city (I'd say the same about 90% of Italian cities). Of course it is better not to walk around showing to everyone how full your wallet is, but keep your purse closed and don't let it out of your sight. This is just general advice that I would give to anyone going to Venice, Rome, New York, Bangkok and so on... I mean, it is really how you normally protect yourself, even in the safest city.
Piazzale Roma is not quite central, but I agree that almost everything is in reach if you go for a stroll. You can also safely ask for hints on public transportation (even if Venice's is quite expensive) to people there.
About Rome and Florence feel free to tag me in your other questions: I've lived in both cities and can help you a lot.