I know you said you've always wanted to join the Army cadet and have friends there, but if Monday night is not good for you, you may want to try out Air Cadet. There are two in the Tri-Cities area (I've been in both) 754 in Port Moody and 777 in Port Coquitlam. Both are excellent squadrons.
754: Meets on Wednesday Night. It's a pretty big squadron and have tons of funding for non-parade night activities.
777: Newer squadron that branched out of 754. I believe the parade night is Thursday if it's not changed. A smaller squadron where everyone literally knows everybody. Less activities due to less funding; sometimes do joint exercises with other squadrons.
I'll tell you my experience about other commitment and cadet. I used to be in a swim club so I know how demanding a sport team can be. I was out of the club when I was in cadet but I still took Chinese school on Saturday. Now most of the "fun" activities are on weekends, the more you go to these, the more you are recognized in the squadron and the higher the chance you get promoted. A lot of times I had to skip either the school or the weekend exercises (some are mandatory, like fundraising). In a big squadron, when you don't participate much (ie. only show up on parade night), no one will know you exist.
I finally quit Chinese school so I could have more time for cadet. When I got to 777, I was in flag party, which meets once a week, and went on every activities possible. Even though I joined cadet (15yo) pretty late, I was able to go to 2 3-weeks summer camps and made Flight Sergeant (3rd highest rank in Air Cadet). I was allowed to challenge a level FYI.
If you are pretty committed to hockey and you think it will take up most of your time, I'd suggest you don't join cadet. In cadet, there is always something to do during the week, band, drill team, flag party, rifle, weekend ex. I've known many cadets who'd quit because they either don't want to try or don't have time for any activities other than parade night. Parade nights aren't boring, but it's pretty much going to classes and it may get repetitive. If you think you can balance hockey and cadet, then I won't stop you. Just remember high school will also be more deanding as you get to a higher grade.
This is just my experience so it may be different for you. Depending on which squadron (or Army corps) you want to join, people will have a different attitude towards skipping parade nights or not attending activities even though you are trying your hardest to make it work.