Oh lordie, YES, giardia can totally mess up the small intestine's flora, causing a plethora of long term problems. However, it sounds to me that you've caught the giardia super early - without even seeing signs of horrible diarrhea - so that's pretty amazing, and lucky. So, I'd doubt the giardia will cause too much havoc...that said, you may want to supplement your kiddo with a probiotic for awhile just to repopulate that small intestine.
His poo could be variable for a while both due to any lingering giardia, and due to both those meds.
As far as schedule, it will be a while before you see something predictable both bc of age and bc of his current issues...but that will all normalize w/ time.
As for the poo texture, it sounds like the slightly harder one is more normal to me...the other description sounds a bit 'mushy' or 'soft serve' (sorry, I know, gross)...but that's prob due to current issue. Poos will firm up over time.
Btw, WELCOME to YT
!