Does anyone know if it is possible to mix different versions of AD&D? I'm hoping to mix Advanced Dungeons and Dragons 1st edition with the Advanced Dungeons and Dragons 2nd edition. What I would really like to know if there are any major differences between the two.
There are major differences, but nothing too insurmountable.
Most 2E DM's used some 1E materials. THe key is picking your core edition, and what you'll cross over. Monks and Assassins crossed over VERY easily. DSG/WSG non-character rules can be used as is.
Again, Pick your reference edition: the rules are subtly different in many places, and it trips people up. So use one as your ruleset, but allow the stuff you want from the others as a port-in.
Also, note that monsters from other 3E/3.5E/PF can be used easily enough simply by looking up THAC0 from HD, and spells directly across by name (even when they do very different things).