Can you mix and match different batteries to best suit available space?
No, you'll get really weird behaviour if some of the cells drop voltage faster than others.
Are Tesla motors the way to go?
Tesla components are an obvious choice, but they're not cheap cars and demand is high.
Leaf's powertrain is limited to protect its battery, since it doesn't have liquid cooling, the cells are quite easy to repackage, easier to fit in a car than the Model 3's four subpacks for example. Ampera batteries were massively overbuilt for what the car needed, capable of really high output, but supply is getting rarer and a bit old.
I think your next step needs to be working out the sizes and shape of the spaces left over when you remove the engine, exhaust, propshaft and fuel tank. Also, capture some CAN logs so you can work out what messages you need to keep the Subaru's dash and body-computer happy.