Is that bad? It's almost impossibe to find one in 1/350. I suppose using 1/300 scale buildings is also verboten...
Nein, das is ja erlaubt!(allowed!) I am not a scale purist and i think using 1/400 ships and buildings would be very fine. I gives a better sense of altitude if ground terrain is a bit underscale. I think that 1/300 would make it look as if the planes are flying too low. Remember that the ground scale in terms of movement and fire range is 1/700 using large ships would then give you problems as it would seem like the range is extremely short for the MG as the ship would be 1/350 scale and MG range would be 1/700.
We are making the papermodel torpedoboat (but in styrene sheets) you linked to, at some point but we will scale the pdf to 1/400 in order to make a model that would look fine in a harbour as well.
Airships in 1/400 would work fine as well i think (but again, i think i would make them in thin styrene sheets).
As for buildings i do not think there is a reason to buy expensive buildings. I suggest making simple models from clay or balsa wood blocks. They look great when looked at from a foot away.
Trucks, Artillery guns and tanks will be available from ColinWe on Shapeways within 2 months i guess.
The church on this pic is made from balsa wood, the buildings in the village are made from ordinary brown modelling clay: