Continuation of Princess Condition

Question
I haven't watered the peanut butter down yet, as she's able to get the small
amounts I give her down without a problem - it was one of the few things
recommended by the vet, along with lunch meat (which it turns out she can't
chew) and very soft goat's cheese.  

She does look a little better today - but I'll confess, my nose is so close to the
situation I'm hardly an objective observer.  

She had an adenoma tumor on one mammary gland, which was removed.  
And at one point she had several pea-sized swellings, that tumor being the
largest, but after the removal of that mammary gland tumor (still pea-sized,
but was producing some milk) they all disappeared.  

They don't believe Princess has a pituitary adenoma though, because she has
good grasping mechanisms still - I'm keeping a close eye on it though.  

I have another question to ask you, if that's alright.  My other rat, Plumeria, is
quite bummed that Princess isn't in the large cage with her.  She's always
been very dependent on Princess, and is definitely the submissive in the
relationship (Princess, though 100g less in weight and smaller, still flips her
over effortlessly).  

I lost my two boys over the summer - the dominant one first.  The
submissive one died of a broken heart one month later to the day - his entire
spirit just screamed out his pain and grief.  

I don't want to lose Plumeriato grief if, heaven forbid, I lose Princess.  Is it
advisable to get Plumeria a companion now, or wait if the unthinkable
happens, and then act?  

And should you say yes, it's a good idea to get a third rat regardless, would
age matter?  For reference, Princess is 1y9m, Plumeria is 1y3m.  

Thank you so much - your advice and support have been invaluable through
this...

Answer
I lost a boy to a broken heart two.....twice actually.  I think I will explain this in case other people read this and say "what are you saying?"
Dying of a broken heart is very real. What happens is, the rat is so stressed by losing his beloved cagemate that they become very depressed. This in turn can weaken the immune system and the rat is wide open for any kind of illness. Also, the rat suffers from failure to thrive, such as loss of appetite which of course doesnt help much if the rat is already   stressed. So the lonely sad little rat ends up passing away, all from a broken heart, so to speak.  I wont say that little Princess is going to pass soon, but if she indeed has something going on neurological such as the pituitary adenoma even if you dont think that is what it is...and lets hope it isnt...but either way, it wont hurt to get another rat now rather than later.

Hope this helps!