Everything you ask for is possible. Probably not within my skillset, though.
can u make a mod "the faster u shoot the zombies, the faster the new zombies spawn"..
It already works this way, for the most part. As soon as you kill a zombie, another one spawns to take its place. If you're set to 64 per player, it ought to maintain more-or-less 64-per-player on screen. It takes a moment to spawn them and of course they have to run towards you, but except at the end of a wave (when there is a short delay) they spawn as soon as you kill them.
With the Infinite Horde modlet, on that last wave they will never stop spawning. Always 64-per-player until 4AM.
and perhaps, when u shoot 500zombies fast, the next zombies will be harder and harder
start night >normal, then green, then police, then terminators and at the end lot of remix and LOT OF DEMOLISHER
This is also how it works already, for the most part. Every higher-GS horde night has a series of waves, with each wave getting harder. Now each wave still will be a randomly-determined mix of zombies, so you might not get "a lot of demolishers", but the chance for demolishers will be higher as the waves get more difficult.
It is possible to increase the chance for demolishers even more, but that is a bigger modlet. Look in entitygroups.xml, scroll down towards the bottom where you see lines which start with:
<entitygroup name="feralHordeStageGSnnn"> (where 'nnn' is some number)
and anywhere you see this:
<entity name="zombieDemolition" prob="
0.16"/>
change that prob value to something higher. Maybe start by doubling it (i.e. from 0.16 to 0.32) and see how it feels.
for 99,999 zombies idont have enough ammo lol..i want only every night +100 zombies (or perhaps a mod that makes every night MORE zombies?, when u reached the maximum of 1350/1400)
Well, it will not actually spawn 99,999 zombies unless you are
extremely efficient at killing them. Basically using 99999 in the modlet just means that the zombies will come non-stop until 4AM. You said you're done by 3AM as it is now. Well, this is going to fill out your horde night all the way until 4AM, with the hardest zombies available for your gamestage.
Edit: Ninja'd by RipClaw