one thing that can get confusing is that the term great harlequin can, and has been used to descibe both the harlequin troupe leaders and cegorach. from the sounds of things:
Among the Eldar it is said that Solitaires are Harlequins who have been touched directly by the Great Harlequin, and have his insight into the fall of the race and the nature of the universe. A Solitaire may live unknown among Eldar (or even members of another race) for years or decades, and there are many rumours and folk-tales telling of Eldar who have met a Solitaire, and realised later that this was the Great Harlequin himself. Solitaires represent the very pinnacle of the Harlequin ideal, and are formidable foes.
Does the great harlequin manage to trick the slanesh out of getting the soli's soul often? It's the Laughing God that does the tricking.
for instance, in his original quote we see the term "great harlequin". from early texts we know that this means Cegorach. the Laughing god.
the latter part of his quote appears to "correct" your use of the term great harlquin... as he may have assumed that it was refering to the great harlequin as the "harlequin master".. or great avatar... or harlequin avatar... which can also be confsuing if simply called "avatar" for obvious eldar conotations.
for the purpose of things, it is easier to use the term "laughing god" or "cegorach" when refering to him...
that being said:
How does a harlie become one?
being "touched" by the
the laughing god is a very vague description. this may lead us to believe that one must come face to face with him/her. or, it may be more spiritual... and it might be that an eldar will be destined to be a solitaire from birth... this may have repercusions upon the following questions. it might well be a mixture of the two. i mean, there are numerous harlequin solitaires and they may have become so in a variety of ways.
the guardians of the black library are supposedly the most ancient and dreaded solitaires. considering the power of the "regular" solitaires as listed in the previous rules for them, i would hate to cross one of those!
Does solitaires have no soul (is it something like a pariah gene effect)?
as kage pointed out, they do have a "soul"... ust that they dont own it. upon death, the soul belongs to slaanesh. though the laughing god may try to trick him into giving it up. being consumed by slaanesh is the worst nightmare of every eldar. the fact that the solitaire fights willing to face this fate is sentiment to his fearlessness. i recal text which supposed that perhaps a solitaire is already dead (which may simply mean as good as dead!?) and this is why a solitaire does not fear it, and so his destruction.
i would not liken it to the pariah gene... but it may be as a result of his fearlessnes. the important note is that the term "soulless" is imperial.
Does the old rules for solitaire reflect this immunity?
see above posts. im unsure if the harlequin rules are still available for download from the gw website, though if you are interested you can search for them somewhere.
Does the great harlequin manage to trick the slanesh out of getting the soli's soul often?
there is no answer to this...
It is said that if one touches the solitaire he/she will be damned, is it true or just an eldar superstition?
this is an eldar superstition. perhaps it stems from the idea that a solitaire is ready to accept death, and does not fear slaanesh. i remember reading a story in which a craftworld was visited by a harlequin troupe... they performed several masques for the craftworlders whom were amazed by the lights and dances. then, from amongst the crowd, the harlequins beckoned forth one of the eldar. he had lived on the craftworld for a long time, going about his business as any other eldar would have... this person was actually a solitaire, and the masque performed the story of the great fall. when the masque left, the solitaire left with them.
although it is said that solitaires live on their own, this may not restrict them from communicating with other eldar in order to go about their lives... blending in. they clearly arent going to get in a discussion about public transport or anything like that... but the above story might indicate that they do enough to hold down a job!