Not modelling in OO (1:76) or N (1:148) scales?

As long as your printer has the ability scale pages up or down there is no reason why you can't build any of the kits other scales. Your printer's 'Scaling' options is usually found in the 'Advanced', 'Properties' or 'Page Setup' section of your Adobe Reader print window. Please consult your printer's 'Help Index' for more details. 

If you are modelling in HO (1:87) or N (International 1:160) all products on can be resized down using table below.

HO scale (1:87 ) - reduce Scalescenes OO to 87%

N scale (International  1:160) - reduce Scalescenes N to 93%

O scale (British 1:43 ) - enlarge Scalescenes OO to 176%

O scale (International  1:48) - enlarge Scalescenes OO to 158%

28mm scale (1:56) - enlarge Scalescenes OO to 135%

TT scale (1:120) - reduce Scalescenes OO to 63%

TT scale (3mm) - reduce Scalescenes OO to 75%

S scale (1:64) - enlarge Scalescenes OO to 118%

Z scale (1:220) - reduce Scalescenes N to 67%

T scale (1:450) - reduce Scalescenes N to 33%

If you are modelling in HO (1:87) or International N (1:160) the card thicknesses can remain as recommended in the instructions.

Excluding HO and International N, it is a good idea to adjust the recommended card thicknesses accordingly.

TT 1:120 scale notes

In terms of the card, the thickness conversions for TT 1:120 scale would be as follows:
Heavy card - 2mm in OO works out at 1.25mm 
Medium card - 1mm in OO works out at about .5mm
Light card - 200gsm in OO equates to about 110gsm
All those card thicknesses or similar are fairly easy to source. Breakfast cereal box card is almost a perfect match for the Medium card and if glued together (as two ply) then you have something pretty close to 1.25 -1.5mm.

O scale notes

Apologies, no plans at this stage for O scale kits. To avoid tiling the enlarged sheets, O scale modellers may find access to a large format colour printer useful (scaled up the sheets are actually closer to A2 594x420mm).

You'll also need to roughly double the recommended card thickness which may require you to print some of the base layers twice. Another alternative would be to use foam core board which is usually available in about 4mm and would be much easier to cut out.