Sounds like the guy at the feed store may not know what he is talking about if that is really what he said. First, I’ll question if they are really leghorns. They may be and they may not from what he said. Leghorns lay white eggs, not brown, so that is a disconnect and causes me some concern about them being leghorns. If they truly are female white leghorns they should lay a lot of nice large white eggs.
And think hard about exactly what he said about meat eaters. All chickens will eat meat if given the opportunity. That’s just the way they are. But if he said Leghorns do not make good meat chickens, he is right. They are small-bodied chickens that don’t have much meat. They are bred to have those small bodies so most of what they eat goes into egg production instead of maintaining a large body. If you want hens that lay a lot of large white eggs, then leghorns is a great choice for you.