I started out with a BPS, moved on to an XT.
I found it helped to have a mid-barrel bead put on. That helped with ensuring my mount was correct.
The leagues I was in were just fun leagues, nothing serious, so they didn't mind if I put a shell in the mag and then racked it in. In other leagues, they get persnickety about only putting the shell into an open action, so you might have to figure out how that's done with a bottom loader.
When I got more serious about trap, I got a gun made for trap. I did pretty well with the BPS but it is at heart a field gun and a gun made specifically for trap makes life easier.
I use 8's and 7.5's interchangeably at the 16 yard line. I don't notice any difference between the two when shooting singles. At handicap yardage, I use 7.5's. I tend to be a slow shooter and even with singles, they are out there a ways before I get around to shooting, so I look for 7.5's first if buying new, but I don't worry about having 8's as long as I'm using them for singles. Up where I am, winter leagues for trap sometimes mean 10 or 20 below zero when shooting and the clays seem to break better with 7.5's, maybe due to retained energy and a frozen target that is harder to break, I don't know.
I started using modified choke but eventually went to extra full as I got better. Nothing like just evaporating the target...