Advice on taking on a slightly older cat than resident cats

I'm looking for a bit of advice regarding introducing a third cat to my little family.  

We already have Kizzie who's 9 years old and Murphy who's 2 years old (both rescue cats and adorable).  We've had them both for around 3 months (not adopted at the same time).

Our dilemma is this - we have been looking at a cat, also with a rescue, who is 3 years old.  We have been advised by some people that it would not be a good idea to get a cat older then Murphy (2 years) and that also some recently neutered male cats can have a tendency, once paws are under the table, to try and take over the household.  We were advised that a younger cat would be more suitable.  Kizzie is never going to like any new cat but Murphy is very laid back and playful and likely to be more accepting.

Has any dealt with this before?  Any advice at all would be appreciated.

I think with proper introductions, cats can learn to coexist together.  Just like humans, cats all have different personalities.  Some cats will get along right from the start, some take a long time and some never really like each other.  I only have 2 indoor only cats.  Last year I did bring a feral into the house and introduced him to my resident cat.  The feral was around 3 and the resident cat was 5.  It was a very difficult transition.  The feral was very territorial and aggressive towards my resident cat.  It took them an entire year to learn to live together.  They now tolerate each other, but are not buddies.  The key is introducing slowly slowly slowly.  If you rush the steps, you will have to back up and start over again.  I rushed the steps and it was so hard.  I thought all was going well and rushed the last few steps.  It set me back at least 4 months. 

The new cat would need it's own room for awhile.  This will allow the new cat to settle in and not be so overwhelmed.  You can start with scent swapping after a few days and then move through the other steps in the links provided below.  Make sure there is plenty of vertical space for the cats that like to be up high.  This can be cat trees or shelving.  Check out Jackson Galaxy's Catification. 

Here are some introduction techniques and a video too.

http://www.thecatsite.com/a/introducing-cats-to-cats

http://www.catbehaviorassociates.com/a-simple-little-trick-to-use-during-new-cat-introductions/

http://www.aspca.org/pet-care/virtual-pet-behaviorist/cat-behavior/introducing-your-cat-new-cat

http://jacksongalaxy.com/2010/10/01/cat-to-cat-introductions/

Thanks very much for the informative reply.  I have been through the introduction stage before (with Kizzie and Murphy being adopted at different times).  I was more interested to know if another cat being older than Murphy would make it worse.  The advice I was given at the shelter was to "adopt younger".  

Really appreciate the reply.

it would take some time for the cats to get to know each others but you should be very careful and pay attention to their reactions when they're next to each others. If they were used to be near other cats in the shelter I don't think it would be a big deal to introduce them and they'll soon become used to each others. because all cats have different personalities they may not become best buddies. My mother have a 4 years old male cat and I brought my kitten to her house and at the beginning the older cat just ignore the kitten and eventually hiss at him. It's been more than a year since then, my kitten is not longer a kitten but a young male and still they aren't good friends, they just ignore each others and share mean looks when they cross paths hehe but they don't actually fight over territory so at least they don't get hurt!