I don't know how to properly structure a tileset in RMXP so that the sprites are aligned with the little lines that separate the blocks :\
I've seen a few HGSS/BW tilesets around, but they're not very big. I'd love to me able to make the graphics HGSS, though :)
And you can catch Pokemon any time of the day; however, in cities, Pokemon only come out at night. There's a lot of morning-only, day-only and night-only Pokemon on routes, so it's recommended to thoroughly search routes every time of day, or else you'll miss something!