Plastic fireforge sergeants, mongols and templars are ok, though templar bodies with chainmail look slim in comparison -frostgrave soldiers wear multiple layers of clothing while frostgrave cultists and ghost archipelago crewmen wear loose clothing. Heads are ok sizewise, fireforge weapons are a bit on the small side (two handed sword especially).
Games workshop brettonians and imperial archers have some fitting parts, but weapons, equipement, heads and hands are generally bigger. There is noticebale difference between frostgrave soldier head in kettle hat and bretonnian one for example.
Games workshop marauders are slightly bigger than imperials and brettonians, but GW heads from all of those kits look ok on frostgrave soldiers and cultists bodies.
Gripping beast arabs are close to fireforge.
I didn't mixed victrix plastics with frostgrave parts, but I think they will be ok.
Warlord games early imperial roman legionnaires and plastic hoplites are smaller than frostgrave minis.
Warlord games Zulus seems ok, same as Perry sudanese mahdists.
Warlord games woodland indians are true scale and are a little taller and slimmer, weapons are much thinner and usually won't suit anyway (mostly muskets), they won't mix well (maybe just for headswap).
With all my love for plastics I've never acquired any of the perry's medieval kits so I can't help here. I also don't know how would those listed kits would match frostgrave barbarians and gnolls.
Caution- I only wrote about size of the parts, bear in mind that heads and hands in all those kits have different joints to the body, so almost everytime with mixing those kits You'll need to make some additional cutting, filing and sculpting (some heads are already sculpted with neck, some kits have neck on the bodies etc).
I cannot help with metals, my limited experience with them is that:
-Reaper metals are sadly noticeably larger than frostgrave plastics and metals, I have two wizards from Reaper and they are larger than armoured knights.
-Otherwolrd minis look ok next to ghost archipelago crewman.