Of course the light cruisers guns can penetrate Tylor's and Oyama's armour. Just think of those unprotected ends and the abovedecks superstructure. A cruiser would get mangled by multiple shells hits and is effectively killed before finishing her off with torpedoes.
And how is that different than the same numbers of light cruisers verses heavy cruisers or pocket battleships? The only thing the armored cruisers (of these types) have over the heavy cruisers is the size and/or number of guns, and it should be able to engage more than one light cruiser at a time, and perhaps a slightly greater chance of surviving the long range fire of the light cruisers and (probably) the hits to the main belt.
This of course means that I should revisit the Oyama design....make her either better armored, or faster (she is armed well enough). Originally there was going to be a step between Tylor and Oyama, but that is being filled by the proposed Atlantean built heavy cruisers. Perhaps their construction will give the Chileans new ideas for Oyama, plus experiance with operating Tylor once she is finished. Or I build a slightly lesser version of it earlier (Libertad), and then when the technology is slight better in the mid-30s, build the Oyama's faster, but just as well armed....so they can keep up with the heavy cruisers/light cruiers.
(The original mess of words this was, is what happens when you are constantly being interrupted while typing.)