Since there's no moving parts in a SSD you don't have to take much care in how it's secured inside the case, so long as it doesn't work it's way into a case fan or something.
I like velcro. Which would have the added benefit of being able to keep the existing hdd for mass storage (if there's an open SATA port on the mobo) since there's only a single(?) 3.5" drive bay inside the case.
Or if you want to be all proper with it, yes, you will need a 3.5" adapter.