@William Redford Nah, you can use the Frostgrave soldiers for unit building, its just that it would likely either be all "swordsmen" without sheilds (ie single hand weapon users) or as miltia if you are just using one box. In fact if Northstar had added one more sprue to the box they could be fielded as bowmen or crossbowmen for KOW out of the box and I know Warlord was selling them along with the other Frostgrave kits as "force builders" for thier own fantasy wargame.
Yes it wouldn't be standardized but thats actually very realistic and common for most sword and bow armies and settings even in fantasy since long marchs, lots of looting, heavy use of mercs,etc. I mean how the heck could they possibly look like a uniform standardized force with all that going on🤣. (The old 48 figure Fireforge Men at Arms kit is what inspired the Frostgrave soldiers set and that’s not uniform either even though it was meant for army building😉).
That said if we focus on keeping with uniformity cause "our fantasy setting has mid-19th+ century logistics due to warp portals and flying ships" or whatever, in those cases all you really need is probably the right head and weapon swap for characters in most cases and even if WA threw in a slightly alternately clothed body it could be used for elite squads .
For example let us say that we have a set of fantasy Zulu wars redcoats themed men at arms whose main builds are guys in sun helmets with halberds, swords, rifles, maybe some crossbows. Well in this case all you really need to make a spell caster is the right sort of arm options (well posed unarmed hands would work, a onehanded staff would work, maybe a hand with a fireball in it) and a mystical looking hat headswap, similar deal with an officer except all you need there is a pistol (you already have swords), for a musician all you need is a bugle/horn arm. If you make some of these alternative arm options for left hands its even useful.
Theoretically one could do that as 4 to 5 body half sprue likely by limiting each weapon option to at most 2 poses, but let’s say this is a single full sprue set and in addition to mention above we want to add a knight/paladin/hero. Well that’s simple, you take one redcoat put an armored plate cuirasses on him and maybe add a cape/cloak to the sprue and an enclosed armored helmet or 2 alongside the mage, officer and bugler, perhaps some armored arms.
Now you may say “but now its no longer uniform is it? and what will you do with the extra bodies?”, actually no it is still uniform with the rest since all you have done is added armor cuirasses over a standard soldier so he is just equipped a little differently from the standard line trooper. You can still outfit him with all the head and weapon options on the sprue, including the other command options (so armored mage and commander +2 bodyguards) and you can also just use the body to make upgraded standard soldiers for squad leaders.
The point is if you’re going for uniformity in your army or war band you want differences between your heroes and your common foot troops to be subtle (ie they need to look like the same army), and that can be achieved best with headswaps and other add on bits to a normal in set soldier body most of the time, and you don’t want to think of special bodies as just hero bodies you need to think of them as regular soldier bodies with new specialist equipment such as armor attached to them in this case.