[SWHouston]

I've always used this criteria...

Same breed,
Opposite Sex,
New animal about two years younger than the Resident,
or is a Puppy.

You'd be surprised how well this works out.
Older animal can really teach the younger a LOT, and
still maintain his "seniority", which is important.
Just about all the work you've done with the resident,
can be passed along to the younger by example.

However, you MUST recognize that there may be a serious
psychological impact on the resident. He will need a lot of
petting and reassuring that he's not being replaced in any
way. This mainly applies when you only have one, and add one.
When there are several animals in-house, this is somewhat
moderated, and the age separation is less important.