First, I'm glad you're not discouraged by your accidents. It takes time to ride well. But, it seems you are having more of your share.
As you have surmised, I suspect there is a mismatch between you and the bikes you've been having these mishaps with. As evidenced as you stated that you didn't have these occurences when on a smaller bike. Not knowing your physical dimensions, it's hard to say theres an apparent mismatch but your experiences indicate thus. Why don't you start out w/ a bike that's a bit more manageable? Like the one you apparently had no problem handling in the MSF course. At least until you can than see and isolate whether the mismatch in the bike was the problem. On the two bikes that you had the mishaps, were you able to (when straddled), put both feet down (flat footed)? Were you able to backup the bike on your own? Were you able to lift the bike up on your own when it fell over? Also, the fact that you accidentally slipped on the front brakes and instead opened up on the throttle leads me to believe that you might not be as a bit coordinated as you need to be for someone to operate a motorcycle safely. How's your hand and eye coordination? Do you play sports? And if so, do you consider yourself pretty good? For me, motorsports is in my blood. I can't explain it except that I just have a passion for it. I gravitate more for motor type sports, whether it's playing MotoGP2 on my PS2 or some racing game in the arcades, that's what I'm drawn to as opposed to most guys being drawn to the main stream sports, i.e., football, baseball, etc. In fact, having no knowledge of how motorcycles operate, let alone, having ridden one, I, at the age of 29 took the MSF course and shortly thereafter, bought my first bike, a 96 Honda CBR 600 F3, rode the spanking new bike from the dealership to my home w/o incident. (A lot of concentration and determination.) I'm just trying to illustrate that some people just have a bit more of a head start than others. I am by no means an expert by the way. What 'm trying to say is that you need to be aware of your own abilities. Assess yourself. Be honest with yourself and ask if your abilities and the bike you've chosen seems within reason. From what you've stated, it's obvious your friend didn't make the right recommendations. Since you're having all these fall overs, why don't you start out on a 125cc like in your class that you experienced and move up once you've mastered the basics and feel more comfortable. Just use common sense. When I took the MSF course, we were partnered up and after the first day out on the course, they had asked my partner to leave because the instructors felt she was not able to master the basics and would lead to her death. They had to make the assessment and they saved her life. The sport of motorcycle riding isn't for everyone. I'm not saying it's not for you; that's only something that each individual need to assess about themselves. If you're still passionate about motorcycle riding, start out small and work yourself up.
p.s. Yes, I can attest to the problems gravel can cause. It's like walking into a floor of marbles. I almost lost it when I was making a turn into a side street; indeed this is where most of the gravel collect. You have to watch these major hazards. It can be scary. One of the commentators on the video, "Motorcycle Survival Skills" indicated that he was paralyzed by this very incident. Please be careful. Also, be aware that most two vehicle motorcycle accidents occur while a car makes a left turn in front of an on coming motorcycle. Be especially cognizant when riding through intersections. Also, be a bit more aware of yourself and your body. I'm only saying this because you had mentioned two incidents (misapplied brakes and having your foot get caught on the pegs preventing you from being able to put your foot down). I almost had something similar. Being somewhat inseamed challenged, I often have to roll up my pant legs up a couple times and on a couple occassions, had that roll up get caught on the foot pegs as you've experienced. Luckily for me, I was able to untangle myself in time. Needless to say, I now always have my pants leg always rolled out. This reminds me of how I see some people ride w/ shoe laces. I see a horrible accident just waiting to happen. Shoe lace gets caught in chain, etc. You can just imagine what would happen on a freeway. Jesus guys, for those of you who ride w/ shoe laces, do yourself a favor and get at least a pair of foot wear that doesn't have shoe laces and I don't mean sandles.
Good luck, Yin.