If they're only 5Ah they obviously weren't intended for starting a car; thus no CCA rating. You'd most likely have to get info from the manufacturer to see if they're suitable for starting use.
Its hard for me to envision that a bunch of those wired together is going to save any room in your trunk compared to installing a regular old car battery or something like an Odyssey which packs a lot of power into a pretty small space. Some batteries are smaller than others, but there is no magic solution that is going to save a lot of room and provide the same capacity (CCA and Ah) if all you're doing is replacing one lead-acid battery with another or a collection of individual batteries. (unless you're planning to distribute small batteries around your trunk into various nooks and crannies and deal with a bunch of wiring)
I would think you could parallel them if they're all new if you REALLY have to go that route.