Is it necessary to replace complete rack-pinion or can you just install new seals when the steering is leaking Assuming power steering fluid seal would not work?

Yes, you can replace just the "boots" on the end of a rack-and-pinion steering provided that the mechanism is not damaged. Unfortunately, the boot is just a dust cover and if the inner mechanism has started leaking there really isn't much that can be done without completely disassembling the mechanism. The fluid in a rack assembly is under pressure from the power steering pump and once it starts leaking the rack is pretty much "done".