I think it should be you need like an arrow an a spider eye to craft one poisonous arrow that lasts for 8 seconds.
The problem is that adding crafting recipes is not as easy as it sounds - the client knows that a spider eye and an arrow is not a valid crafting recipe, and no matter what the server tells it, it will deny it's a valid one. The only way to truly "add an item" is to change the way the server deals with an item without the client having to do anything "new", and as for recipes, your client needs to recognize this recipe, which it won't by default, thus why servers usually use commands like /shop or something to shop for items. Let's take MCTIMV as an example:
In MCTIMV, after collecting a few "traitor points", you are able to buy an item from the "shop". This way of collecting/"crafting" items works because it is accessed through commands - the server is doing all the work, calculating your points, taking them away when buying something, giving you the item...your client is fine with all of this because the server is doing all the work. All the client has to do is make the items show up in your inventory and send the information/command to the server. In the server, there is "player files" or whatever you would call them. The client is designed to "download" this information from the server. Within these files is things like your inventory, the armor you're wearing, the stuff in your enderchest, etc...when you buy an item, it simply adds that item you wanted to your playerfile, and very shortly afterwards, your client "downloads" this information from the server like it is designed to do, and like magic, the item is in your inventory. (At least, this is very similar to what it does. The exact details I'm not sure, but either way, your client is completely capable of tasks like these without modifications.)
Now let's take MCTIMV's creeper arrows as an example: You are given 4 creeper eggs when you buy them, which act as the creeper arrows when shot. When you fire an arrow, the client tries to suck up an arrow to fire it, and it does, but the server just adds another arrow to compensate. The server now takes away one egg, and that arrow fire is now considered differently by the server - when the arrow hits the ground, the server spawns the creeper there as if a spawn egg was used in that spot, and so no new items have to be used for your client to do this - as far as the client cares, an arrow was fired, an arrow was deducted, and a spawn egg was used...all stuff it knows with no mods, no extra textures, no extra files/scripts, etc. It's all on the server, and that's what a plugin is...something that edits the way server handles certain things, not the way the client does. (This is actually a slightly inaccurate description of how that plugin works, I know I'm wrong at least somewhere since you can't even pull back the bow with no arrows...but it's just an example.)
...Was it neccessary for me to explain all that to you? No, not really, but this is just something I thought other people in this thread might be interested to know - until the modding API for Minecraft comes out, you cannot simply do things like "add a crafting recipe". Your client is simply incapable of it without downloading a mod prior to joining the server. (which a server with the crafting recipe added would require you to have a client with said recipe installed onto it to connect.)
Poison arrows are certainly possible, but they'd have to take the place of another item and could not be crafted, sadly. (Unless you used another crafting recipe already existent and made the crafted item the edited one.)
Back to the topic:
I agree with some of your points but i dont understand the part about having to do 5-7 hearts of damage. When i said it would slow them down i meant it does the same damage as a normal arrow but just slows them down and not doing damage.
Yeah, that could be done by applying the slowness effect...I thought you meant actually poisoning them, like as if they were poisoned by a poison potion, which does damage to them. (slowing them down like fire would.)
