@peemee Yes and no: it'd be risky brewing Earl Grey and other oil-flavoured teas in them, and you do need one teapot per style of tea (I have one for red, oolong, and pu-erh, though some people go further and have 2–3 for oolong depending on style). They usually come in 300ml max — 150-200ml is more common — because they're for gongfu brewing.
If all you're after is heat retention, consider a tetsubin.
// @streakmachine