If that is one of the blown packs (an inch or so thick) they are not the best option for DH use. They hold the tank above your back an excessive amount and do not allow the tank/reg to be set as low as it should be. What you want is one of the thin (about 1/4 inch) packs, the USD Kam-EX for example. They are much better suited to DH use. The blown ones are fine for single hose use.
As for which tank you can use, some of the later blown ones had straps that would accomidate both sizes. I have swapped the straps from those to the thin packs to use larger tanks with. You can also get SS standoffs and extend the bands to accomidate the larger tanks