what do you mean by internal? do you mean in the header? If it's in the header then you should be fine but it depends on how you add to the header.
What you would ideally be doing is adding it to the wp_head using the wp_head filter e.g.
function mytheme_styles() {
// stuff here
}
add_filter('wp_head', 'mytheme_styles');
Do the check for the post type in there (using the WP conditionals) and you should be ok.
Another way is to have another CSS file which is enqueued when only on that posttype but would only use this way if the css is quite large.
Finally, you could also add it to the normal CSS file as WP does (or can) output the posttype into the body class, so you can do .posttype-name .element
in the css.
I personally prefer putting it into the css files but it's down to how much css you are using.