This question might have been dealth with before but I would like to have a response since I'm new here and it is the first time I'm working with different map scales in a project. The maps include land use, land form, gas lines, electric lines, topography, etc. The scales range from 1:25000 to 1:1 000 000. Can anyone show me the best (most reliable and scientifically defendable) method to put them all in a database with scale at 1:25000? This is not merely for display or presentation purposes.

Generally, combined data is reported at the lowest scale (1:1M), not the highest (1:25K in your case).

For instance, TIGER is advertised at 1:100K, even though some of its source data was 1:24K, other source data was 1:100K, so the resultant product was 1:100K.

Are there only a handful of features at 1:1M?

You could maybe list your source layers and the scale of each.