If I have myself a modded xbox as the host and play of the harddrive from say a game such as halo 2. As other people without modded xboxs play on dvd, with there downloaded content from xbox live will that cause problems? What if the host has all the downloaded content will that still cause problem? If I install the halo 2 multiplayer map pack on the host xbox will I have to install it onto other xboxs to lan them all? Finally if I put modded maps on the host xbox, will everyone else be able to play them?

Everybody has to have the maps to play them. You have them and somebody else doesn't, you won't be able to play them.