I think if your spare cage is tiny for a GCC, then it would still be a little too small for most other smaller birds. If you are looking for a bird smaller than a GCC but with a somewhat similar personality (smart, fiesty and usually cuddly) a parrotlet comes to mind, but even though they are smaller than GCC's, they are very high energy and need a large cage for their size.
Other small birds to possibly consider are budgies or a handfed lovebird, but again, even though they are smaller birds, they need a cage nearly as big as a GCC.
I know many people are saying to wait for your GCC to settle in, and that's a good idea, but there are also benefits to having two birds - mainly that they can help keep each other company when no one's home.
If you get a new bird, you would need to keep it separate from your GCC for a quarantine; after that, they could be in the same room, in different cages. They should not be let out at the same time without close supervision, however. Whenever you have birds of different species and different sizes, there is a risk of serious injury if they fight.
But before you get a second bird - a few things to think about:
- Both birds will need attention and out of cage time, if they don't get along well, they will each need separate out time - do you have enough time to give each bird enough attention (they will help keep each other company, though)
- Both birds will need vet care, food, toys, cleaning, ect.
- Both will need decent sized cages, even the smaller birds should have a cage of 30x18" or so, bigger if you can. Your 'tiny' spare cage might be too small so you may need to get another. The spare cage can make a nice travel cage, though.
- There's no guarantee each birds' favorite person will be the person they belong to, a bird you get for your dad might turn out to love you and hate him, ect. Babies, if tame, tend to be ok with everyone, they will usually develop likes and dislikes later on or become aggressive with some people, good socialization will help minimize this.
