Oh dear-Here I go disagreeing with the mods on my first post
In my mind 650 points in 25mm on a 6` x 4` table is by visual perception no means a wargame, It`s a skirmish.
Hannibal you seem to want to view your figures whilst playing so forget 6mm unless you have the eyes of a Hawk.
15mm on a 6` x 4` table will give you a decent balance between ground/figure and base scales. <--- This is always the problem with wargaming so identify what your criteria are first.
If you want to see your lovely painting then do 25/28mm figures, But you will run out of room on a 6` x 4` so to go to the other extreme and you run 6mm figures that you cannot see (If your my age) then they can run around all day and never find a melee. 15mm works fine on a 6` x 4`.
So inside your question is to put smaller scales on larger bases---------Try one unit because it looks awful!!!! Of course you can lower the figure to man ratio and mess with that but if you go to a club you will be out of scale and look odd.
So as a final piece of advice-----------15mm on a 6` x 4` with these rules looks and feels fine 6mm on these rules will look awful unless you change your figure to man ratio and that leaves you being the only person in your area that you can fight without looking like David to his Goliath.
Dare I say this that I think FOG was written with 15mm in mind so bear in mind that your perception of what you are doing is actually going to change beyond reason in my mind by adopting a smaller scale figure. If you want to do 6mm look at Warmaster rules.
Well that is my 2 groats worth. And I have not even played a FOG game yet!!!!!!!!!!!!!!!
Nitey nite--Being about the only Brit still up at 4.20AM.