All very possible, and in small model, desirable. Double pole switching is overkill, you only need to switch one wire over. If you can stand the weight, 5 might be better for a bit of extra performance (it is a Vosper), but either way, the only tricky question is whether or not they will stand supplying the current. Totally agree with the comment about good soldering, not so much from the potential for fire, 250mAH would probably struggle to light a ciggy, but reliability is the issue. If you make it up as a sausage string, it might be possible to service the pack (i.e. replace it when the batteries have passed their sell by date) without destroying the model.