Do we have to pass through customs when arriving inVienna from Paris?

You should not have to go through Customs, unlsess you have anything to declare. Just walk on out after collecting your luggage. As for Passport Control or what others call Immigration you will process through at CDG.
Enjoy your trip.

1) You will not have to go through customs, because both France and Austria are in the EU.
The EU is a Customs Union, no customs between EU countries, and no limit to what you can take from one EU country to another.

2) But more important, you will not go through immigration (passport control), because both France and Austria are in the Schengen Area.

1. the transfer co., Wien transfer, asks only for the airline and flight time. Will the driver consider customs ( if necessary) and baggage pickup, so he doesn't have to wait too long?

The company will be aware of the rules and regulations, and now how much time to allow.
But, as above, there is no customs (but even if there was, customs takes zero time). And the thing that could take time (immigration), isn't there either.

3 If we just get a taxi, do the drivers speak English?

They don't need to, as long as you have the name and address of your hotel written out.
Make sure you write the address out exactly as it says on the hotel website (or better still, print it out).
Addresses are arranged differently in different countries. If you change anything that may be the important bit.

Thank you, both for your helpful reply!

The drivers of the taxis speak good enough english. Save ten or 15 buck and actually get there faster and easier in a taxi.

When I’m concerned about potential language barriers I print out the hotel’s address. Often the hotel’s website will have a directions tab that will suffice for printing. If the address seems too small for easy reading then I copy and paste it into a word processor and increase the font size. Show the address to the taxi driver and say “bitte”.