a) 6 is the maximum, 3 the minimum for requesting fic, so that our Beloved Request Sorters have a good chance of being able to match you to someone who wants to write that which you have requested. Request too few and they might not be able to find you a writer. Request too many and they will reach for the whiskey bottle and be unable to match things. (It's an optimisation problem involving subsets and links, if you're into that sort of thing.)

And because they enjoy my pain and tears as I cut my list down from the 15 I really really wanted :-)

b) Yes. Absolutely. If you want to receive a story about the soon-to-be-raised 11Foot8 bridge, or the 1938 Robin Hood movie, or even the Real Women of NASA, then request away. It doesn't matter if you didn't nominate them. If someone nominated them, then they want to read or write the fic for that fandom.