Hi there! welcome to the wonderful wacky world o sonic. Sorry your questions are just being answered now.
1. Your best throw in most situations is upthrow. (Downthrow at high percents, for mixups, or tech chasing fast fallers) your combos will of course vary depending on percent. at low percents on most characters, upthrow - dair leads into re-grab until about 30% (on average). from there soft aerials chain into others, or you can dair into tech situations. upthrow - dair - uptilt - dair upair - dair *repeat* (i call it the Dunkin Donuts :3) is my personal go to. also Upthrow to fair/nair/dair can be a good finisher depending on DI
2. Side b is a fantastic move. in neutral, it can be used as a bait (side b wavedash back, etc.) or as a burst movement option (akin to sheik or peach's DA). on hit confirm, you can cancel anything out of it. your best followups are usually JC grab or RASCAL, but i personally like to go for soft aerials or BA first for added damage depending on percent. Side B is also an amazing tool for just dragging Characters off the stage for a gimp or edgeguard situation. (this works especially well on spacies)
2. Down B is interesting. You don't get much of raw down B. Most Sonics use it as a cross up into aerials (down b usually puts people in the habit of shielding, making Down b - Cross up - dair a decent bet for a shield poke.... however, its obviously a good idea to just go in and grab in this situation too. I personally go for Down B (spindash) in situations where my opponent is backtracking or feels uncomfortable. for example, if you're fighting a DD heavy marth, on his dash back you can intercept him with Spindash into a free aerial/knock down. Also, as simple as it might seem, Spindash is just downright fast. most people feel intimidated and have a hard time keeping you out, making it a great tool to pressure unconfident or inexperienced players. however, be careful not to become too comfortable spinning to winning. spindash can be difficult to control, and in a lot of cases, you'll lose stage control, putting you in the position to get bippity bopped. Also a lot of times, sonics become locked in specific timings and habits when using spindash. so make sure it always has a purpose! i should also mention that Spindash connects into nair at high percents for a go to kill option.
If you have anymore questions, please feel free to message me or hit up the sonic discord! (List of discords -
http://smashcords.com/brawl_pm/) Hope i could help ya out.