Bashish is compatible with ordinary bash prompts and also introduces it's own generated themes with the .bt extension.
Zshprompt can co-exist with bashish, just set the theme to none and use zsh's own prompt loading functions (you still get themed applications)
Unfortually prompts from the old BashPrompt project will not work in bashish unless modified, here is a code example of such a prompt:
{
PS1=foobar
}
{
PS1=foobar
}
foobar
unset -f foobar
Note that bashish makes some things in the HOWTO redunant (eg. title changing, see 6.1.6), and also is incompatible with the use of a theme function.