So I am new to this server. Joined about a day ago.

I've already begun construction of my house, and i've got plenty of supplies of cobblestone and dirt, sand and glass, etc. But one thing i'm planning to do in the near future is construct a hostile mob system so that I can get a good supply of bones, arrows, string, Iron, exp, and in game cash.

So... what are the rules with that kind of construction? will I be able to get a world guard on it?

I can't say anything about WG, but try not to make it too big, else it will get destroyed :)

What laegluin said and don't afk next to it and make it so you can turn it off when you aren't using it.
Unfortunately lots of mobs causes lots of lag (minecraft just can't handle it, a better server won't even make it faster :( ).
We usually do worldguard it :]

I had thought of that already, and had planned to work around it. The location I was planning to choose would be some common ground-level Y coordinate (like 60), then I would dig a 2x2 hole down the center as the elevator, and make pads below that area where mobs would spawn while I work up there.

The design I have in mind, unfortunately, is easiest to shut off using lava and pistons. Incorporating redstone lamps would make it a lot more clunky, but I suppose I could manage. or ask for special lava permissions.

Mob grinders cannot be built in city plots. They must be built out of the way in the wilderness.

Just one suggestion for your trap: Make the elevator smaller and burn the spiders, that solves a lot of problems :P