Good safe answer Don.Ras.

But to get on target there is as always more informastion required:

1. What is your level of competence in LPG engineering?
2. What is the connected gas load (m3/h or kW) and how is it distributed along the manifold?
3. What are the lengths of pipe involved for each of the sizes refered to? A dimensioned sketch would be very helpfull especially if it included the information from point 1 as well.
4. What is the required operating pressure for the load it is usually 38mbar for most commercial and domestic cooking/catering equipment?
5. What is the ambient temperature range in the area where the bottle banks stand? Temperature and particularly solar gain can cause all sorts of problems with LPG storage. Too hot and the bottles exceed safe max prressure and too cold you have to add evaporators to do the phase change from liquid to gas.
6. If the bottle banks are correctly sized for the load then good engineering practic requires that for a first stage or rough-cut regulator be installed on the outlet of the bottle bank to reduce the pressure to less than 2 bar and that should be followed by a second regulator dropping the pressure down to the pressure required by the appliance manufacturers for the safe operation of the burners.

In answer to the question at the end of the second example:

Yes, the density will change approximately in line with the combined Boyles & Charles Laws. P1V1/T1=P2V2/T2.

15 PSI is a collosal amount of pressure for a kitchen... In the UK LPG supply pressure after the PRV, governor, UPSO's and OPSO's is 37mB, thats about 0.54 PSI, I'd seriously check your facts or leave it the hell alone. On the other hand you could fry an egg pretty damn quick!!!