Thank you, gents. You are all very helpful.
Drilling more holes would be my last resort, as I suspect ropes would be good enough for me. I tested my DIY outboard rowing rig a few days ago and roped wood boards worked reasonably well; only the right side got loose at bottom. - adding ropes there should fix it. I asked here, because I want to see different design configurations.
I understand my aluminum canoe can take lots of screws in lots of places. Indeed, I just drilled 6 holes on it yesterday to install/try a second outboard rowing rig design and to secured my first outboard rig (which revealed 2 loose c-clamps out of the total 6 clamps in my test - so I replaced those 2 with screws).
I prefer ropes, also because we may have 0-2 rowers on the canoe facing either stern or bow. My two DIY rowing rigs allow that flexibility (and I have tested the builds in person). I may have to drill at least 6 holes and install 3 sets of rails, if I do it that way.
Ropes are more flexible, if they work well enough for me (I will test them).