dnj30:
also, how long a hose should i get for my primary in order to be comfortable sharing in an ooa.
I assume you are wearing a Bouyancy Compensator of some kind. What kind of BC are you using? What kind of diving are you doing with it? (What type of rig does your buddy have?...e.g., are either of you uncomfortable doing OOAs at the moment?)
If you are doing recreational diving, there is nothing at all wrong with your hoses in your current format. A 30" hose is adequate to be handed off; as it will be if it's your primary and your secondary is some sort of Air II or the SeaQuest or Atomic variant. Just as you were instructed, in the event of an OOA you and your buddy rendezvous, someone grabs someone somewhere, s/he gets your primary, you go to your secondary, you both ascend, hopefully together. What you have should work comfortably for both of you. No worries.
You could put a 5' hose on your primary, but many BCs with cumberbunds, integrated weight pockets, etc. don't offer a suitable position in which the long hose may be secured yet re-stowed without assistance.
If your's does, great. Some do, some don't. It all depends on what you have for a set up currently.
The thing about a system is that the parts are integrated in a manner that all the elements of the system are mutually supporting. That is what makes it a system.
If you pick and choose different aspects of the system to add to your current rig, this is not "bad" - you may be more efficient or effective than you were previously.
The point is, however, that the result is unlikely to work together in the same manner as the entire system does, because you've isolated one part of it and tried to add it to something else.
If you'd like to add a long hose to your rig, go ahead - a 5' hose should be adequate, running under your right arm, up across your chest, around the back of your neck and with the second stage into your mouth. You'll find that some way to secure it (where you can restow it yourself) may help you to keep it from going adrift at extremely inconvenient times.
Depending on how your buddy is equipped, how you and your buddy train, what you have for a BC, etc, however, you might also discover that its more of a PITA to deal with than what you're currently using.
FWIW. YMMV.
Doc