Ok, I own a female (I think) Cavy named Lucy. She was a rescued piggy....saved from being dumped in a woods and living in an uper tiny cage that was only cleaned every two weeks, eating cat food, and barely able to get to her water. Her is a pic of her shortly after we got her:

(She is now off of the Cedar bedding and we were the ones who got her the house and the Piggy food. That is the peice of junk cage came in.)
Anyways, I have some questions (and each of the questions have mini questions in them).
I've had Lucy for about a year or so, but I don't know how old she is. When we got her she was an adult, but I don't know how old. Her owners didn't know how old she was either, as they got her from someone else. Is there anyway to tell a Cavy's age?
She is also EXTREAMLY fat. I don't know what I did wrong, but she has gained a BUNCH of weight. When she sits and is hunched up in a "ball", she is around six inches wide. Her diet is a pellet diet with vitamen C added to it (around 1/4 a cup a day), a large handful of Timothy Hay, and some various veggies, probably only 1/4 a cup a day. Yet she is still fat! I don't understand.

Anyway to slim her down?
I've read that Cavies do better in pairs. If I were to get another one, I want to make sure it is of the opposite sex and that they'd get along. So how can you tell a female adult Cavie from a male? Do the male's testicles drop, or not? That would be the easiest way to tell, because she doesn't have testicles. lol
She has been a single Cavy for her whole life...would getting another one upset her or do they take to other Cavvies naturally? What would be the best way to introduce two Cavies? Right now she is in a cage that is....4'x2', I think. Is this big enough for two Cavies?
Sorry for all of the questions!