All in the title
Lizards are the best to have in small hearths, as for the main old ancient hearth it is situational but usually I have a harpy in the ancient hearth and if not a harpy then a human and if not a human then a beaver and if not a beaver then a lizard. Leave choices for the players to work on and manage. Don't automate the game please. At the setup screen you can perhaps add an (ancient hearth tender) drag and drop bubble choice if you must do something about this. Or a preferred starting hearth tender drag and drop 1 bubble representing each race.
They already do, unless you have Harpies. Harpies > Lizards when the game places a race in the hearth, and also usually in general.