I am trying to sort out the details for our 1 day cruise stop early November in Athens.
We are a group of 5 adults but with different degrees of agility. Some of the group have been there before.

Looking at taking the metro in from Piraeus to either Monastiraki or Akropoli stations. I gather it is about a 15-20 mins walk to the metro depending on where exactly the ship docks.

I understand Monastiraki is in Plaka. If walking up to the Acropolis, which station is best?

If part of the group wanted to take a taxi to the entrance to the Acropolis, is this reasonable? How much walking would it save?

I can't sort out exactly where a taxi would drop you. I hear some bad stories of Athens' taxis. Last time we were there we used a taxi once and I think it was ordered through the hotel. Approximately how much should a taxi be for 3?

Although taking the metro is by far the most inexpensive way to get from Piraeus to the Acropolis ( about 1.20 euros) you have to keep in mind the metro stop at the port is at the ferry docks not the cruise ship port. It will take you at least 20 minutes to walk from there to pick up the metro.

If you decide to take the Metro, both of the stops you suggest are about equal distance to the Acropolis however they are both all uphill and will take at least 15 to 20 minutes not counting the time it takes to navigate the narrow streets of Athens.
Actually getting lost and figuring out how to get to your destination is part of the fun of Athens but you only have one day.

Given you are on a short clock, I would think that using your energy walking to and from the metro could be better spent at the Acropolis and all the antiquities and shopping in the Plaka.

I would suggest you seriously consider taking a couple of taxi's to the Acropolis which will drop you off pretty close to the ticket office.

I see you have heard about the Athens taxi drivers. For sure I have felt pretty ripped off as the drivers inexplicably charge you more than the meter reads and who who mysteriously can't speak a word of English at that point. However I stopped being paranoid about that when I was able to put it all in perspective. Shortly after coming home from Greece I had occasion to take the taxi home from the Calgary airport. It cost me $35 and it is an easy drive on freeways all the way. The taxi from Piraeus will be in chaotic city traffic, it will be in a maze of narrow streets and the car will be a stick shift. You should expect to pay about 20 to 25 euros for the ride. I realized I was probably getting a much worse deal in Calgary than Athens, so I have decided to happily pay the driver for getting me safely to my destination and I even smile and give him a tip. Enjoy the experience.

It was years ago that I went to the Akropoli. I took a taxi from Omonia and had him wait while I climbed up the hill to look around.

Mmm yes I too probably would be inclined to take a taxi as unscrupulous as they are . Otherwise from the Monastiraki metro stop go to the flea market nearby where you'll find the entrance for the Agora and Acropolis. We didn't need to queue and you'll find the Agora interesting too. However if some of the group aren't agile this plan could be very tiring.

Take a taxi John - just to save your precious time- and energy. We were in Piraeus on a cruise 2 years ago and asked the taxi driver to take us straight to the Acropolis. It cost 30euros for the 4 of us.He drove us right to the gate so we were well pleased. I remember walking up the hill in the heat on a previous trip to Athens and thought I would faint with the heat.
Much easier to save the walking for the downhill part!

Look up Athens in www.portreviews.com for all the info needed for the train into Athens and other very useful information.