Its best to start your own thread as this one is old and the OP hasn’t been back on since. It’s also easier for us to offer you ongoing advice and support when it’s your own thread. I’ll ask for it to be moved.
Sadly introducing a single piggy to a bonded pair is one which can fail When done on spec. When adding piggies its best where possible to date your piggies at a rescue centre so they can choose their own new friend.
A single piggy needs to live side by side other piggies for constant interaction through the bars. Living above will reduce the interaction greatly so isn’t really a long term option.
The minimum welfare cage requirement for a single piggy is exactly the same as the minimum for a pair (be that two sows or a neutered boar with one sow) - that is both a single piggy and a pair needs to be in a minimum of 120x60cm - so as long as her cage is that size, then the cage size is right for her having a friend.
If she has a friend living in the cage with her, then the cages absolutely can be stacked one on top of the other to save floor space.
Please do consider dating her at a rescue centre so she can choose a character compatible friend. She may be making her own fun, but the issue is, as a lone piggy she would have no choice but to do so and its not necessarily because she is happier alone. If she has always been alone and never learnt the vital social skills as a youngster, then it does make things a bit harder so this is also dating her is going to be the safest way to find the right piggy for her. They will help to find a piggy with whom acceptance occurs and you may then just need to bring them home, pop them somewhere neutral for a few hours (bathroom floor, kitchen floor, bath tub even) before transferring to a clean cage. Or, some rescues offer on site bonding so you would take her there for a few days and then once she has found a friend, bring them both home together.
https://www.theguineapigforum.co.uk/threads/guinea-pig-forum-recommended-rescues.196734/