1. If I were to get yellow perch, how big should I expect them to get? Should I expect over a foot, or would they stay smaller in an aquarium setting?
That depends upon how much you are willing to stunt them. Let's be honest, that's what we are doing when a wild-caught fish fails to achieve it's potential size in a tank. Intentionally or not, we are manipulating conditions...food quality and quantity, water parameters, levels of accumulated waste, available space and a host of other factors...in a way that keeps them from growing naturally. That's stunting.
2. Would a 60 breeder - 75 gallon be good enough for these? If so, how many perch?
They're an active, schooling fish in nature, easily exceeding a foot in length. How many can be kept in a 4-foot tank? Again, how much stunted growth is acceptable or hoped for, and how much natural activity and behaviour is desired? Perhaps, I would answer the "how many?" question with "zero".
3. Is it possible to keep them alone?
Possible? Of course it's
possible. But...desirable, or attractive, or a satisfying experience? Only you can answer that.
I kept Perch many years ago, as a group in an 8-foot tank with very few other tankmates. They were vivacious, active, attractive and fairly fast-growing.
I've also seen single Perch kept in tanks as small as 20 gallons; they weren't any of those things.
esoxlucius
would have some insights on this species; his Perch are Europeans, not identical but very similar in almost every way.