Still pretty new to this game. What level should I have my characters at relative to those that I'm fighting? Is it safe for me to pump all my 1 and 2* training droids into my main characters?
Also how big of a boost is promoting a character up a star versus getting them up gear levels?
Plan on being below the enemy level pretty often IMO. There are a couple stumbling blocks in PvE light side/dark side missions that may "force" you to out-level them (or bring in ringer allies to carry you) but most can be done underleveled. Ditto that with PvP/Galactic Wars.
Grand scheme of things, the order of importance is generally:
Gear > Levels > Stars.
Gear is the only way to increase (meaningfully) the majority of your stats that aren't HP/Damage. Your speed, potency, tenacity, etc. is solely dictated by your base-stats and your gear. Increasing stars/levels has no impact.
Levels tend to have more of an impact than Stars, but it's also much easier to increase your level and it depends on by what measurements you're going by.
Going from level 60 to level 70 is pretty comparable to going up from 6* to 7* (IE, a level 60 character at max gear at 7* is very close to being equal to a level 70 character at max gear and 6*).
To put it into a better perspective, going up 10 levels or increasing your stars from 6 to 7 is generally a ballpark 80-100 more damage and 800-1000 more HP.
This is why my general advice to new players is to stop starring guys at 5-6 stars depending on their purpose. There's a pretty large diminishing return for your investment getting that last star (or even going from level 68 - 70 or something which costs a coupld hundred thousand of credits). IMO, you should probably only level heroes up to the highest "gear" tier they require... that'll also save your droids/credits in the long run and let you work on secondary characters.