For Middle Republican, the cohort was a flexible organization, but started at 6 centuries and would include triarri - an impermissible battle group in FOG.  The other alternative is an entire acies of hastati or principes, and FOG's list makes this impossible by declaring the units to be mixed.  The discussion of 'legions' in the FOG list is not consistent with the scale, and I take it as saying you must have either BGs of 4's or 8's not an attempt to describe any real organization. 

 

The BG concept works very well top down for most battles but will - in its abstraction to create a game across 100s of types of armies-  have to adapt a fair bit to get an exact replica for any army known to be that specific.  Although anytime I have wanted to do that it hasn't proved difficult. 

 

The nominal scale you mention is very secondary to the shape and feel of an army design, and it is the scale rather than shape that should typically give when doing this.  As the rules are working games between armies that at times numbered 100000 vs others that never reached 10000 this is the starting position.  Don't there fore feel too bound by the nominal scale as it is shape and feel that dictates how and army runs in FOG.   My Early Samurai army to true scale has a base being less than 50 men. 

 

The BGs are troops locked together under a local (unrepresented) junior general for the purposes of a battle.  This pattern is consistent in most battles for all sizes of armies, and therefore a better "solid" pattern to work from - hence the above philosophy. 

 

I am not an expert on the Romans specifically but it seems pretty likely that in detail 1 - 3 cohorts may be given a job together under one commander and 4 - 7 another etc. and in a large game the BGs represent such sub-blocks and the sizes may well vary from 4 - 8 out of proportion as cohorts are bound together for a purpose.

 

 

FWIW I use 1 8 and 4 x 4s as a legion representing the double size cohort roughly in proportion.  And have 2 of these in my Late Republican design for a double legion army at 1000 pts.  The shape feels correct on the table and it looks great in 25mm.

 

There is nothing to stop you putting a single legion out literally as organised.  Again as a rough version try 1 x 8 and 9 x 4s and 1 Cv BG plus some Auxlilia and LF and LH and then you will be roughly the shape and feel of a single legion in FOG perhaps with 10 BGs as cohorts in this case.

 

So overall feel too bound by the nominal scale - which is why we called it that and didn't labour it.  Rather feel free to creatively adjust it to specifics to get an army shape and feel that fits any expert knowledge you may have (and share it with us in the army design section on the web-site too if you can).

 

Cheers

 

Si