Since many have answered your questions, I'll just offer my two cents.

For a couple of years I had been wanting to get Kia a companion but never really researched or thought deeply about the ramifications of having two dogs.

I don't regret adopting Chipper and he's really settled in and has melded into the pack so to speak.

I do walk them together but because they are about the same size, they are not too difficult to handle. Chipper likes to go, go, go! While Kia wants to stop and sniff. Since I want to go, go, go... it has now become two against one, so Kia begrudingly caves. I enjoy walking two dogs more than I ever enjoyed walking Kia alone. Because Kia never wanted to walk. She just wanted to stop every 5 feet to sniff and pee.

Chipper came with a nasty habit of jumping and suddenly, Kia, who never jumped up before, began too.

So in a way, Chipper did teach Kia a bad habit which we are working quickly to nip in the bud. However, teaching him good habits has been pretty easy since Kia is there to demonstrate. He picked up shake in less than an hour.

It's sort of a give and take relationship. Chipper was enrolled in obedience and we pretty much have been giving Kia a refresher course. It's been a good experience so far. Though it's funny when I practice a sit, stay with them. Chipper will obey, while Kia will come slinking over after a couple of minutes with this "Are we done yet?" look.

She must think since she's older, she can get away with it.

Kia still has mild food agression but Chipper doesn't try to take away treats from her. Just patiently waits to see if there will be left overs. And she has been letting him lick her empty food bowl, something she's never allowed my sister's dogs to get away with.

They play great together, but he does get on her nerves sometimes, and that's when we intervien with a 'time out'.

Definitely has it's pros and cons, but I can't imagine being a one dog household ever again.