Can I replace my 1TB HDD to 256 SSD (2.5")?

All standard SSD's are 2.5" form factor. The one in the guide is a special Apple drive that you would have a very hard time getting.

You can still do this, by replacing the existing 3.5" drive with the 2.5" SSD. All you need it an adapter kit. I'm not sure if iFixit sells them, but I know OWC does.

http://eshop.macsales.com/item/NewerTech...

There are other ones out there, but be careful, not all of them are designed to work with the iMac's mounting system.