I have a 7-year-old female cat, adopted from the Humane Society last September. About a month ago we moved from a two-story house into a smaller one-bedroom condo. Perhaps because I brought a lot of familiar furniture, she seems not to be very anxious in this new place, but she's become rather inactive. She has perches to look out the windows, but there's not as interesting of a view as there was at the last place. She doesn't play with toys unless I'm controlling them, and even then she loses interest pretty quickly, and I'm not always home. She has also become pickier about her food and water - she won't eat dry food any more, and she doesn't drink water any more. I'm afraid that she's understimulated.
For this reason and others, I'm considering, in the near future, adopting a second cat. I understand that a young male cat would be the best choice in this situation. But I don't know whether my cat would actually get along with another cat. At the house we lived at previously, she and the neighbour's cat would sometimes fight at the window, and those are the only interactions between her and another cat that I've witnessed - but they were never properly introduced or anything like that, so I don't know that I can predict how she would interact with a new roommate based on that. I don't want to go and adopt another cat and have her hating me and the intruder forever...
My question is: are there any behavioural signs, that I can observe without having another cat around, that might indicate whether she could coexist with or even enjoy the company of another cat? Am I misguided in thinking in the first place that adopting a second cat could be a way to bring some life into her?