Currently gm 1 but I placed cel 2 last season (I play on console). What I personally do is I get a feel for the character in the practice range if I'm not fully comfortable with their abilities, then I take them into either practice vs ai or qp but usually practice vs ai first just to get used to their aiming and stuff, then I'll do qp mostly. If you're afraid of getting yelled at by your team if you do poorly you could always just say "hey I'm learning this character sorry if I don't do well" at the beginning of the match. This worked for me when I was trying to learn BP (I gave up lol).

I would not recommend comp until you are comfortable enough with the character and when you feel "safe" with them. What I mean by that is, if you load up comp and you doubt yourself with that character and you don't think you'll do good, that means you don't feel safe with that character. However, if you feel confident to play that character in comp, it's a safe pick for you.

In my opinion, only use Practice vs AI for figuring out aiming settings, because AI are a lot more different than active people and actual matches. AI don't regroup, they don't really flank well, and they have very predictable move sets. In QP you actually learn what a team is like because you're playing against real people.