A:
The 2008 Imac used 3.5" desktop HDDs, SSD drives are much smaller. This drive is smaller so to fit properly in the cage you'll need to get a 2.5" to 3.5" adapter rails. Another option, you may be able to use velcro, double sided tape, etc to secure the drive to the inside of the machine. Another annoyance to some Apple iMacs, some use a HDD vendor specific thermal probe that's built into the factory hard drive. So if it came with a Seagate drive for example, you'll need to plug that Seagate thermal sensor into the replacement seagate drive otherwise the fans will run at 100%. Of course, WD drive versions use a slightly different plug too.
Since the SSDs do not use that "feature", your fans will run at 100% unless you bypass that with a software fan controller running in the background on your machine.
Then again, you may be lucky and get a thermal couple that's taped to your drive; in that case, just tape it to the outside of the new ssd or cage and not have to worry about the fan annoyance.