I was able to defeat an immortal dwarves combo in a 3-playergame with one new player ( so no help from him).
The key was killing his race in decline so that he had 'only' 11 points a turn. Hide on the other side of the map and try to accumulate more than 11 points a turn. Do not fight them!!!
It's a good combo, but it only gives them map pressure. Just go ...
The fact that there are 10 is the good side.
This is similar to other races from the base Small World and its expansions. For example, ratmen get a larger number of tokens with no downside. Kobolds (never leave a kobold token alone), barbarians (no redeployment) and pixies (only 1 stays on the board in each region) all have even larger token numbers but ...
I am unsure about an 'official' list (i.e. from Days of Wonder), however I can list the races and abilities that would not be transfer as well to the other game. Page 10 of the SmallWorld Underground rulebook does offer some general advice if you are aiming to play with SWU races on a SW board or vice versa:
Powers and effects that make no sense (because ...
From a literal reading of the rules, I believe your friends were correct; you do not get the discount when attacking a non-adjacent space with Flying Doormat.
The Cultist rule says:
Conquer any region adjacent to the
Great Ancient (this includes any Region
that may be adjacent as a result of any
Racial, Special Power, Popular Place or Righteous
From the rules (p.7)
Once a player's conquests for the turn have ended, he may
freely redeploy the Race tokens he has on the board, moving
them from one Region to any other Region occupied by his race
(not necessarily just an adjacent or contiguous Region), provided
that at least one Race token remains in each Region under ...