Hello
@Pippa!
I wouldn't consider myself a Zebra Finch expert. I only have my two and they are pets, but I can give you some advice that helped me with my pet zebras
I was told that you either want
2 birds or 6+. If keep 3 one can get singled out- 4 and they pair the two groups can fight- 5 one is singled out.. 6+ there are enough birds for social dynamics to not be as polarised. Zebras have babies easily and you will easily have a whole house full of them if you don't take care in which birds you place together. Male Zebra have the orangy cheeks- some mutations have slightly different hues- but females will not have coloured cheek patches. If you keep a pair male/female you shouldn't give them a nest or anything they could make into a nest. They really can have large clutches and it is not an exaggeration that you could end up with more birds than you can handle (hundreds), so please mind this advice.
I keep two males, one was a rescue and he has a bad wing. The other I got from a breeder but looked for a relaxed bird who would be patient with bird who can't fly as well. You want to do your best to select birds that have a similar personality. If one bird is a very active go getter, get both birds that are active go getters- a more timid/placid bird with a more active bird can be stressful over the long term. That said if you keep 6+ than they can figure out who they like on their own and the personality matching is less of your responsibility.
For diet they require seeds. They should have access to a finch seed mix 24/7. I also give them a leafy veg to pick at -they really enjoy pea shoots- but this is more for enrichment than diet. Seeds are their food! Too much veg can give them loose stools. Ensure that you have as many food ports or dishes as you do birds and possibly even an extra dish - so if 2 birds 2-3 eating ports or dishes. You don't want one bird to control the food supply.
The males sing often and I enjoy their little voices. Very bright and chirpy
Though not everyone likes them, and some find their voices a bit annoying- so check with your grandmother and ensure she enjoys their song. Females do not sing to the same talent as the males.
My two makes sing all the time and each one has a unique song. It is wonderful. Keeping males together is easiest for me. They are very close friends now, and preen each other and do activities together. I hope my experience helps give you some guidance.