8 weeks is still pretty young. Is he fully weaned? When I was breeding budgies, I made sure my chicks were able to sufficiently feed themselves before letting them go to their new homes because 7-9 weeks is a pivotal age. If they wean themselves and then relapse, you have to re-wean them which can be a tough issue. If he's not fully weaned, *demand* that the guy wean them before he sells the bird to you.

But back to your original question, you can't let the new addition share the same cage as Spike because you'll need to quarantine the new bird for about 3-4 weeks. If you don't quarantine, you'll run the risk of giving a fatal illness to Spike. After 3-4 weeks, you can start to introduce the new bird to Spike's cage. However, if the new bird is of the opposite gender, they will breed in the spring. I can almost guarantee that