So might be doing some long-term travelling across different cities, countries, and thinking of bringing a 55L backpack with normal clothes etc, and then having one full hiking outfit to do some hiking aside from general travel

For day hikes I would use a 15L backpack and just put a down jacket, fleece, hardshell, and hydration bladder inside for the day.

For multi-day treks would it be possible to use the 55L with the one hiking outfit in and attach a sleeping bag to the outside (or put that inside and use the daypack on front with extra stuff) or is it too little a size? Won’t have a tent and food won’t be cooked, and if I did any extreme hikes I would rent a backpack and only bring hiking equipment (such as going up a mountain on a tour). Although I could potentially attach a tent it depend.

As a side note is 15L too small a daypack for hikes.

Thanks.