Red stairs are for special (non random) levels. You'll need to enter in to know if they are worth it. Just avoid Unholy cathedral, which is a melee-only level (unless you play melee, of course). For the other, you'll get some stupid death, but this is kinda necessary in roguelikes.
My advice is to try them asap, since ignoring them looks like an important handicap to me (less xp, and you lose rewards).
Angel are challenges with restriction, just to make the game more difficult. In other words, finish the game normally before trying them.
Your kill percentage looks correct, and good.
I don't pay attention to score. It depends on challenge type, difficulty level, kill percentage, levels completed...
To find informations, the beset source I know is the wiki.
http://doom.chaosforge.org/wiki/DoomRL_WikiSince it also contains spoilers (special level maps, assemblies, monsters stats, etc.), you might prefer to avoid it until you know enough of the game. it's up to you to choose. (Personally, I consider "spoilers" are just game's rules, but opinions highly differ about this :)
Of course, the forum contains a lot of informations about many topics, including the sticky "quick question threads".