I
really like your Charizard. I think you've got the right Idea on most things, and I REALLY REALLY like the upthrow to side-B combo you did against Luigi. I've tried that every now and then, but it's hard to judge which characters it works on. I think most of all you need to improve on landing your nairs and auto-cancelling them a bit better (sometimes your nair landings are a bit akward) and also sometimes you threw out nairs that missed and left you vulnerable.
For the Shulk MU, you should play defensively and punish for counters. Shulk's moves aren't all that fast, so waiting for an opening should be your priority. Also stay away from those annoying F-smashes. They can mess you up.
Toon Link on the other hand, you want to play aggressively IMO. You don't want to give TLink room to breathe so he can pull out bombs and boomerangs and all that. You want to get up-close and overpower Toon Link's short sword range. Constantly expect Toon link to down air whenever he is above you, so wait for it to happen and grab. Don't charge an f-smash, because the windbox on Tlink's down air can possibly push you out of range.
For Luigi, I believe you should play defensively at first but slowly become more offensive. I believe that at lower percentages, Luigi can be more of a threat purely because of how if he manages to pull off a trade or a read and gets you into a combo... it won't be pretty. Use your mobility to get around the fireballs, and ALWAYS respect the luigi missile. NEVER doubt the missile, because when you least expect it, your opponent will get a misfire.
Lastly, For diddy, you want to play a mixed style. I've played against a couple of diddy kongs, and what I would do is play aggressively at the start to throw my opponent off. I like to take diddy's Bananas and shove them in his face. A common diddy strat is to try and get those up air follow ups off of down throw. Use your Charizard mobility to jump out of those dumb follow ups, and retaliate with a falling nair or something. If diddy tries to leap at you, a well timed side-tilt should do the trick. I don't have too much match up experience with diddy, but you should definitely be careful.
I haven't really mentioned flare blitz much, because I figured you can integrate it into your playstyle however you want. Use it to primarily Punish rolls, recover, and for follow ups on some moves. You can even use it when the opponent is firing a projectile. One cool trick with flare blitz that you should only use once per game, is when you're recovering at high percentages, flare blitz onto the stage, but then do another in the opposite direction. It will catch people off guard, because their only thought is to set up for an attack after you land from flare blitz. Hope all of this text wall helped, and feel free to PM me or something