See the attached for a comparison. From left to right, you've got a Lamming knight, a Julie Guthrie Grenadier half-orc (one of the larger Fantasy Personalities), a Lamming Viking berserker, a Ral Partha cleric, a Lamming Viking warrior, a Ral Partha shield maiden and a Lamming wizard. The Lamming Vikings are exactly the same size as their
As you can see, the Lamming figures aren't massively different in height from those old Ral Partha figures, but they're quite different in proportions. Lamming figures mix well with Minifigs; they're in a very similar style.
That said, I absolutely would use all of these together in a game; at that sort of scale, you're going to struggle to achieve the sort of close compatibility you might get with modern 28s. And I'd note that the Lamming Viking and the Ral Partha shield maiden would look fine together: sexual dimorphism is real, after all!
I've got a few Tin Soldier UK human figures. From what I can see of those, they're more in the Minifigs/Lamming mode than early Ral Partha (stockier, bigger heads, etc.). Again, I'd use them together.
One thing maybe worth noting is that there just aren't many metal figures in the "25mm to the top of the head" bracket (unless you're talking about 1/72). My understanding is that the petiteness of those early Ral Partha figures is the result of a mistake by Tom Meier: he thought that "25mm" meant "to the top of the head" whereas other sculptors had always used "to the eyes" as the height marker. That's why Ral Partha figures quite quickly increased in stature - there's a real jump between the first "fine" Tom Meier figures (i.e. the 1979 ones, not the cruder Wizards, Warriors and Warlocks range from 1978) and the slightly later ones, which are 25mm to the eye.
I'd understand "true 25mm" to be generally used to contrast with the creep into 28mm. So Lamming, Minifigs, early Citadel, etc., are "true 25mm" whereas figures by Essex and later Citadel, among others, that were being sold as "25 mm" in the 1980s, were actually much bigger - so not "true 25mm".
Given that early Tom Meier was out of synch with everyone else, I don't think there's much in the early, idiosyncratic Ral Partha size out there.
Hope the photo is of some use!