Some of the outfits can be mixed and matched, such as the Luchadores mask with the Deckers Specialist outfit, or the female warrior pants with one of the normal bra's or tops. But no, you can't use Luchadore trousers with a hoody, the luchadore trousers are a one piece.
As for the dlc packs, they take place within the SR3 story but they're like side stories.
Clones touches on what happened to Gat but other than that, the story in Clones could be taking place anytime in the SR3 timeline after Gats Death, while Gangstas is pretty much the same. Both dlc storylines are more about the Saints recruiting new members, with Clones its about getting the Gat clone as a Saint, and with Gangsta's its basically Jenny becoming a Saint.
I can do without the clone of Gat but Jenny is a crazy, homicidal, bad ass. So I'm really hoping she's in SR4 sans the movie costume.
Between men and women there is no friendship possible. There is passion, enmity, worship, love, but no friendship - Oscar Wilde