well, the general concepts of
40k vs. 30k that
FW gave out were:
both sides use the 30k
CAD and force limits (no
LoW's under 2k, 25% rule, only troops score (i'd give warriors and raveners implacable advance), no formations (though I would probably limit him to 1 formation since 'nids are pretty week
ATM) allow him to field up to double the normal maximum unit size (i.e.60-strong squads of 'gaunts

) These apply to basically every
40k army and it makes the game a bit better
imo (tones down the stronger armies while not really hurting the weaker ones, making the playing field a bit more level)
The megarachnids aren't actually in the rules, so nids are just a rough approximation. if you want to fluff in nids just go with them representing non-humanoid xenos races that were purged