All the Caldari COSMOS agents located

I've thought of this myself after realizing that my gallante standing was -7 (running with a Caldari mission corp will get you there pretty fast), and eventually decided to just pump up my social skills. Its quicker then trying to get your standing up, and unless you've completely buggered your standing with a faction you shouldn't be killed on site if you happen to wander into space claimed by a faction that doesn't' like you too much.

The only time I'd worry about having great standing with all four factions is if your character is going super hardcore into industry(and I mean 100% non-stop indy character). If you're in EVE for the pew pew, just keeping your faction standing around the -2/3 area for everyone is good enough to not have your ships popped on sight should say, a war target fly through space that you'd get popped on sight for.

Eh I wouldn't say that Sisters are neutral, or at least they weren't a few months when I played last. They give you gallante with minor bonus to minmatar.

Can't say that I've ever ran into factions that were neutral. If your'e trying to keep your totals with factions from being lopsided, only thing I could suggest would be grab a minor corp (aka no navies) with decent lvl4's in the amarr/caldari, and the gallante/minmatar camps and mission back and forth.

If you decide to do so, be carefull to accept only the missions where targets are pirates, or non-faction ships, else it would be a huge blow for your standings.