Agreed Kyrt. This precise question of mixed barrage and other weapons was raised in the past. Neal's answer was to dice for and allocate the barrage hits first, then allocate the hits from other weapons front to back to those units that have none.
Remember, unlike normal shooting, the hits under the barrage template are applied to the unit diced for. So the 'convenience' is to dice for groups of units of the same type under the barrage, applying any hits gained to those particular units. However, the hit allocation process for normal shooting is intended to spread the hits across all the units front to back. By adopting the above steps this even spread of hits is achieved.
As to different ranges for the different weapons, I believe each weapon is restricted to its particular range. Otherwise, units like the Leman Russ would be able to fire Heavy Bolters up to the range of its Battle Cannon, an obvious nonsense . . .
So you can only shoot the barrage weapon if it is in range to an enemy unit, at which point the template is placed over that unit (and as many other units as possible).