The seat needs to be very snug just sitting in it. On the track you need that tight fit and you won’t notice the tightness. I loose seat in a 125 shifter is not good and could bruise your hips from moving around.
+10 on the seat good advice here. I can barely fit in mine but then as soon as I get moving I don’t even notice.
Run c12 you’ll be glad you did. When I first started w the S-M 125 I used 98 pump gas and after 6-8 weekends had a detonation ring which made both the head and cylinder worthless. That was over $350 to replace. I generally run 4-5 15min sessions on a practice day and rarely use more than a gallon to gallon 1/2. At ~$20 a day in gas for the c12 it’s a good investment. Plus if you’re planning on tuning a pwk/pwm with the right jetting race gas will help a lot in giving you stable readings.
+1 on running C12 since you can’t go wrong there. I ran “100” octane from a local 76 station pump which wrecked the piston and cylinder from detonation in only a few sessions. Turns out that fuel has a 95 motor octane. Recommended motor octane for these engines is at least 100 to prevent detonation.