How do I calculate the XP for these races/classes now? I am wonderin if I can still make them multiclass still as well?
Thanks in Advance!
My interpretation...
The old Grandfather Rules basically said anything from AD&D (or older, in this case) use those rules, but integrated into 2e.
For example, let's say I had a 1e version of the AD&D Monk (1e PHB version, not the OA version). I would basically transfer the entire class (including XP table) into my 1e game. However, I would try to use as much of 2e in it as well (so, probably use the 2e THAC0 table if one could...but that would leave the question whether to use the Rogue or Priests THAC0...in theory, using the old to Hit table would work as well, including the THAC0 connected to it's calculations instead).
Basically, it said anything from 1e that you were already using could be ported over to 2e.
However, it would not matter which TSR version or edition you were using, if the PC's were a different race than a Human and were single class previously, they would not be able to be multi-class after the initial character creation. The only exception to adding a class after Character creation was the 1e Bard.
The only version which allowed Multi or Dual classing after the fact for Elves, Dwarves, Gnomes, Halflings or Half Orcs (And other races beyond Human) later would be 3.X editions and later.
That doesn't prevent you from going ahead and making them multiclass if you feel that fits the character ideas better, but there aren't really any rules for that (and hence you would need to make your own house rules, which is something many would say D&D is all about anyways) and so you would be the one who would need to decide how that was done.
If I decided to do something like that (I normally would not), I'd probably take the XP that the PC's had previously, take the two (or three) classes they wanted it applied to in 2e, and split it accordingly as I would with any other PC in 2e. That would redefine what levels they were at (and accordingly, be restricted as per 2e level limits) due to how the XP would work at that point in relation to their new classes. This would probably also mean they would be reduced in their Thief class levels, but they would also gain new abilities in a new class as well). (Most likely 16 would be the new Maximum Level limit for a Thief if they had a 19 DEX, though with the new class and XP split, for the 17th level Thief this may already be the case anyways. For the 20th level Thief, they probably would lose some of their thief levels, but would gain some power otherwise. I would suggest they have their second class as a Mage or Ranger as those have the highest level limits and for the 20th level character, may have the biggest benefit for them...as long as they had the pre-requisites for such a change).
The other way I might imagine to do it is if I use the optional idea of doing away with level limits, in which case the suggestion is requiring 2x - 4x as much XP per level to increase a level. I'd probably require 3x the amount of XP per level (starting at level 1) which means that the characters would probably lose a few levels if we utilized that method for a multi-class character, but the abilities gained from being multiclassed probably would outweight what they lost in power as a singular class.