I'll Echo the sentiment that I've done tons of Canadian twist-offs with my bench capper and never had a problem although it takes considerably more pressure to cap twists compared to pry-offs.
2 possible tests to determine if the caps are sealing properly:
1. Seal an empty bottle and hold it under water for a few seconds if you see any air bubbles then water is getting in and the bottle is not capped properly. Now open the bottle it should be dry inside.
2. Fill a twist-off with Soda to the same height you would beer and cap it as quickly as possible, leave it for a few days and then uncap it, if the soda still has some carbonation the CO2 didn't escape and you were capped properly and should be good to go.