Is there a way to pattern molding against an uneven floor, using plastic molding as the trim, similar to cutting tile pieces for not full pieces?
This is between drywall and a concrete floor in the garage.
I'm not sure what you mean by "similar to cutting tile pieces for not full pieces" however, if you are asking how to handle gaps under baseboard molding because the molding is not flexible enough to bend to cover itself, the solution is to use a 2nd piece of more flexible molding over the first.. This is easily accomplished by using "shoe" molding" which would run along the base of the baseboard molding but is very flexible and can follow the gaps in the floor.
Hope this helps!