Welcome to EM and to the Forum
I’ve completed my whole journey with contact lenses only, so yes, it is not a problem to use contact lenses. I appreciated the extras added from the peripheral vision. Or in other words just always hated glasses.
(Obviously assuming your eyes are healthy and you can wear contacts comfortably but by the sound of it you are OK with these.)
As for the different strength for glasses and contacts, the -5.5 glasses are about the same as the -6 + 1 = -5, i.e the contacts plus reading glasses combo.
It is because of the vertex distance: contacts sit on your eyes directly while glasses are at an average 12 mm away. This is only important above -4D.
-7 opto style overcorrection with glasses matches the -6.5 contacts and typically there is an average 0.5D overcorrection by optos correcting people to 20/15 or 20/13 or 20/10 visual acuity.
So the diopters you mentioned sound about right to me.
One thing though: at one point you may consider purchasing prescription reading glasses online from the likes of Zenni or the local online equivalent in your country instead of using cheap reading glasses from the stores. Just for the better quality. Still cheap online.
This is the official conversion optos use:
This is the visual explanation: