So I am converting Against the Slave Lords to PF1 using slow Ex Track to deal with different ex tracks from the day and to add to role play. Has anyone don conversion for some of monster and deal with Multi class demi humans.
Multiclass demihumans are probably best dealt with in PF by giving them a class which combines the two - magus for fighter/magic-user etc. I haven't done such conversions but I could if you want to pass me a few details.
Thinking fighter thief being a slayer, theif cleric being an inquisitor. It it also monsters like mudtigers in A0.
I would recommend rather than trying to do direct NPC conversions, determine the CR you want your NPCs to be, and then pick appropriate class/level combinations from there. AD&D adventures had NPCs and monster all over the place in terms of power level--you could easily run into something that would be a challenge for 5th level characters to fight in a 1st level adventure just because the designer thought it would be fun to put it in there. And maybe you want to do the same thing, but otherwise I would suggest choosing what CR you want, then choosing class and level from there.
If the NPC is not going to be in combat and/or not plot essential, I would suggest just picking an appropriate prebuild from the GMG or Villain Codex or NPC codex (I pull from the Archives of Nethys' NPC index, especially for something like merchants).
THAT SAID:
IIRC (based on an old conversion guide I cannot find), the formula for switching multiclass from AD&D to *3.0* (not PF) is take your highest level class as a base number, then take the rest of your classes and add those together and then divide by 2, rounding up(?), and that is your class level. Then rebuild your character with those class levels any way you like. So a level 3/2 thief/fighter -- you take 3 levels of thief as your base of 3, then divide the fighter levels by 2 (1) and add them together -- you have a 4th level character. You could rebuild this character as a fighter 2/rogue 2, fighter 1/rogue 3, fighter 4, rogue 4, or even just build them as a totally different class (e.g., slayer) if that feels better.
Further for converting 3.0 to Pathfinder, as a general rule of thumb I would make every character 1 level lower than the direct 3.0 conversion, because Pathfinder classes are more powerful at low levels than in 3.0. So that fighter/thief in the above example who came out as a 4th level character, I would actually build as a 3rd level character in Pathfinder (and again of whatever combination of classes you like).
For monsters I would simply find a comparable monster in PF and substitute if you don't want to homebrew from scratch. If you want to homebrew from scratch follow the monster building guidelines in the Bestiary.
I think the only one that stands out in the adventure from a monster conversion is the Mudtiger. That was a unique monster to that module and I don't recall seeing a 2.0/3.0/3.5 conversion for it in D&D much less PFE1. Pretty cool monster as well. If I recall it was a outsider (earth elemental) lizard/alligator type creature with the ability to shock people from range and then if they have ferrous material it becomes magnetized. Cool little creature and they are a pretty significant part of the adventure.
Jack