Legions at the start

Without downloading any mods, is there a simple way of starting the game with Legions?

Yes.

Open the export_buildings_descr.txt file and do a find and replace:

Find: "and marian_reforms"
Replace with: ";and marian_reforms"

This will allow you to build the post reform units.

Thanks. What function does the " ; " perform? (and / or)???

In coding, the semicolon just means that the program won't read that line. So by putting a semicolon before the and marian_reforms line will make the program not read the restriction that the unit can only be built once the marian reforms have occurred.