Hey!
I've got a few question that I hope someone doesn't mind answering.
of course not

How long has this mod been underdevelopment?
Uhmm. Don't really know, I'll let the devs speak

Does it have a very big team?
Idem

Have any of them worked on previous mods that have been released?
Of course, Rival Species part 1! check this out.
http://www.rivalspecies.com/Is it just going to be half life 2 with 40k models or is there going to be significant changes to make it more unique? Are there going to be any 'special featurs'. What kind of player load is the game being aimed at, and as such how large will the maps be?
It will not be a "basic deathmatch", if that's what you're asking.
The gameplay has been discussed on this forum (you can find the threads easily): it will be a team,objective based fight in wich one team is composed by the tactical Space Marine squad, while the other team is composed by the Tyranids. The Tyranids players will be the synapse chreatures, capable to direct and command a small brood of AI bots.
Surely some other one will explain you better, since I'm a bit in an hurry

Anyway, welcome!