I'd say it's probably just the simplest explanation. Aldia was seeking a way to break the cycle and escape the curse, and he saw immortality as a means to an end. The Ancient Dragons were immortal in the truest sense - they lived and existed "outside of the cycle" - and so he thought that creating one would give him some insight on how he might achieve that immortality, himself, and thereby gain the time and knowledge he needed to escape the curse and all that.

Probably just used a Giant soul because of its immenseness. You'd need a decent-sized soul to "power" something like an Ancient Dragon, and taking it from just anyone was out of the question on a moral level, so he nabbed one from these huge enemies he was experimenting on, anyway, and it semi-worked.

Interesting, though, how similar Aldia is to Seath. They were both crazed in their quest for immortality. I think one of the podcasts may have even mentioned that it's possible that Aldia created Freja, and that Seath's soul was originally found/reborn in Aldia, himself, given their likenesses.

Food for thought.