How would an 11 year old beagle terrier react to another dog being placed in the house?

depends on dog. he might not like it. introduce slowly and don't leave alone.
References :

I would let him pick the dog, Try to find on who is around the same age as he is and very submissive.
References :

If they meet outside away from the house , then you can see if they can actually get along . But if you just bring the dog home and place them together then its mostly likely going to lead to fighting . And you should always whach them together for awhile since he is new to the house hold and the bigger dog can easily hurt the smaller one .
References :

It's my opinion that behavior problems (especially dog aggression) should be worked on BEFORE bringing another animal into the home. I would bring in a behaviorist for dog number one…. Dog aggression could be inherited or learned, so what's to say the same thing won't happen with dog number two?

There is no way to know if your dog will accept a newbie or not. You could try to do a neutral territory introduction but that does not mean your dog will be okay on HIS turf OR over any period of time. I would seriously have to ask if it where worth the risk, both for stressing the older dog out and potentially putting the younger (new) dog at risk of injury.
References :
Owned by Mutt

I wouldn't do it. Your dog is 11 years old and has been the only dog for all that time. Unless the other dog is needed as a Service Dog, I would suggest waiting to get another dog. It wouldn't be fair to your dog.
References :
de

Hi! Hope this helps.

Resident dogs don't always relish sharing their territory and human family with some upstart new pup. To make sure peace and order prevails, take some care in introducing your new dog or puppy.

Click the link below for more information.

Thank you!
References :
http://dogtime.com/new-dog-old-dog-training-center.html