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Foundry do some nice Numidian cavalry, whih are slightly smaller then Victrix, so should probably fit well. They also do some Greek cavalry in the older range. Again should fit well as should the Macedonians especially the Thracians.Warlord Games have taken over the Warlord Factory lines but I am not sure if the Gallic cavalry are still availabe. Ebay might be your friend here.
You have lots of Gallic cavalry from Foundry and they are the same size as the germans.https://www.wargamesfoundry.com/collections/gauls
Could still use victrix Numidians. They are considerably smaller than the rest of the Victoria cavalry. I am in agreement with you - Their cavalry is ridiculously oversized for 28mm even against their own infantry. But their Numidians were done right.But since you're trying to match the forces you already have, Foundry is probably ideal. You could head swap some of the victrix Numidian cavalry too if you want to convert some greeks.