Can a non-expandable filter be used in place of an EZ flex furnace filter?
As long as the replacement fits without gaps (which would allow air to bypass the filter) will be "OK" to use.

Whether the replacement filter meets your needs (besides cost) depends on the effective size and screening size.  It may be hard to compare these factors with discount filters

The effective size of a deeply pleated filter means you get a larger filter that will clean longer.

The screen size of the paper relates to how small a particle the filter removes.  Its a tradeoff, because super fine filters (pollen level) will clog faster.