First, welcome to TRF! We are glad you joined in here, hope we can be of some help too
The sort of things you are asking are the same qstns lots of other people ask. The short answer(s): there are usually several ways to do just about any of this.
1) Elastic v.s. none --- For many low-power rockets, with a long enough piece of kevlar (or other good-quality, high strength fiber) for a recovery system anchor line, like about 3 x the rocket's length, the ejected nose cone will probably slow down enough that when the line is fully extended and the nose cone "snaps" to a stop, you won't have much of a problem. Why is it so long? You don't want the "snap" to be so severe that it yanks the anchor line from the nose cone (separated NCs are seldom ever found) or that it yanks the other end loose from the attach point inside the rocket. If you have lots of ballast weight on the NC to trim your rocket for flight, the "snap" load on the attachment will be more severe. It's just an easy thing to do to add a section of elastic (some folks prefer the rubber-band-like material) to give a bit of shock-absorber-action to the entire system. Possible problem with this approach: if you select an inappropriate ejection delay and the recovery system deploys at high airspeeds, the opening jerk of the parachute can tug hard on the kevlar anchor line, which can cut into the front cardboard edge of the body tube and cause a lot of damage (this is called a "zipper").
2) What length --- You will get many opinions on this one. Some people like to use elastic for the full length of the anchor line. I happen to like to use a kevlar anchor line attached to the rocket, with a long enough length to reach well outside the front (so if I have to make repairs later I can reach the knot where the kevlar connects to the elastic), usually using a piece of kevlar (or similar) that is about 2 to 3 rocket lengths (this cord usually packs easily into a small volume). I tie on the elastic band, and you may want to add a snap swivel at this connection for clipping on a streamer or parachute. The elastic band should be about 1 rocket length (or not much more), with the NC attached to the far end, so that when the whole mess is in descent mode (hanging under the recovery device) the NC is suspended at an intermediate point and is not banging into the rest of the rocket.
3) What goes where -- I hope I covered this in the explanations above.
4) 3 x length elastic --- If there is room inside the forward BT, more elastic (and kevlar) can be helpful. At some point it probably becomes just dead weight to add more length of elastic (like, it doesn't really help much to add more), probably a length of elastic around 3-4-5 x rocket length becomes wasted effort. If your rocket only has a small volume available to pack in the recovery system, you might have to skimp a bit and try a shorter length of elastic.
5) Why elastic at all? -- Again, if you just used a long piece of basic anchor line (like kevlar), as long as it will pack into the rocket and deploy without getting all tangled in a giant knot, you might be able to skip the elastic part completely. If you try using a line material like nylon (woven cord, like surveyor's line) it will have a little stretch already (kevlar tends to have very little stretch or "spring"). However, with materials like nylon you will have to begin worrying about the anchor line being damaged by the high ejection gas temperatures.
Some rocketeers like to build (especially on heavier model rockets) with stainless steel woven wire that attaches to the rocket and has a loop on the free end to tie on the rest of the recovery system. If you would like to try this, check out your local sporting goods store for fishing leaders. This might be over-kill, but at least you won't ever ever ever have to worry about ejection charges roasting and severing the anchor line.
The problem with the shock cord materials supplied with many kits from "big" manufacturers is that this is the only anchor line material in the kit (they don't include a length of kevlar) and what they do provide is too short. This results in an ejection event where the NC runs out to the end of the stretched elastic and is immediately pulled straight back into the front end of the BT, giving you a nice dent in the cardboard. You can fix it by straightening out the BT and reinforcing with a few drops of superglue (dollar-store kind is OK) applied on the inside. Most of the newer kit manufacturers have listened to the customer complaints and designed their kits with better materials, and you should not have this problem.
The most important point that I would like to make is that the attachment at the rocket-end needs to be a little more carefully thought out than many kits show in the instructions. A certain major manufacturer of kits likes to use a "tri-fold" system glued inside the front end of the BT (deep enough so you can still seat the NC). I don't like this system because it means adding a thick wad of folded cardboard in the path of the recovery system where something can get snagged, torn, or completely plugged and stopped. (This is not a good thing.) If you have kevlar available, even a minimum-diameter design can be "fixed" by tying the kevlar anchor line to the thrust ring before the ring is inserted and glued in place. The kevlar anchor line then runs the length of the rocket and extends out the front and goes on to the rest of the recovery system. If your rocket has a larger BT, with a motor-mount tube (MMT) held in place with centering rings, you can tie the anchor line around the MMT and hold it past the outside edge of the forward centering ring as you insert and glue the MMT assembly in place. This gives you a solid attachment for the recovery system and lets you pack the anchor line behind the parachute, which I think is a much better approach.
And then there are always the practical considerations, like if you only have certain supplies on hand and want to be able to launch tomorrow (without waiting for a mail-order), you use what you have. Hey, this is supposed to be fun, not work.
As you can tell, this is one place on the internet to get your questions answered 
