How do I italicise individual words within \scshape

Question

I am currently using mathpazo for the font for my thesis. I want to use smallcaps for my section headings (as you can see in the MWE I have set \allsectionsfont to \scshape. The problem is that I have some botanical names in some headings that need to be italicised. These include the Tolypothrix and Calothrix in the MWE below. Does anyone have a working solution?

\documentclass[12pt,a4paper,openleft,draft]{report}
\usepackage[lmargin=4.0cm, rmargin=2.5cm,tmargin=3cm,bmargin=2.5cm]{geometry}
\usepackage{mathpazo}    
\usepackage[margin=10pt,font=small,labelfont=bf,labelsep=period,singlelinecheck=false]{caption}
\usepackage{sectsty}
\usepackage{setspace}
\usepackage[english]{babel}
\usepackage{slantsc}


\allsectionsfont{\scshape}
\doublespacing
\begin{document}

\subsection{Differing results from phylogenetic analyses have significant consequences for the placement of Tolypothrix and Calothrix in the Nostocales}



\end{document}

Answer 1

Since, as David says, there is no small caps slanted or italic font, I'd use the "poor person's small caps" solution: uppercase letters in smaller fonts. In the example below I used relsize to get matching sizes; probably some fine tuning would make the match better:

\documentclass[12pt,a4paper,openleft]{report}
\usepackage[lmargin=4.0cm, rmargin=2.5cm,tmargin=3cm,bmargin=2.5cm]{geometry}
\usepackage{mathpazo}    
\usepackage[margin=10pt,font=small,labelfont=bf,labelsep=period,singlelinecheck=false]{caption}
\usepackage{sectsty}
\usepackage{setspace}
\usepackage[english]{babel}
%\usepackage{slantsc}
\usepackage{relsize}

\allsectionsfont{\scshape}
\doublespacing
\begin{document}
\thispagestyle{empty}
\subsection{Differing results from phylogenetic analyses have
  significant consequences for the placement of
  \textit{\smaller[1.5]\textlarger[2]{T}OLYPOTHRIX} and
  \textit{\smaller[1.5]\textlarger[2]{C}ALOTHRIX} in the Nostocales}



\end{document}

Answer 2

The OP's question is about italic small caps in sectioning headings, which by default are bold. Boris' answer -- although the faked letters are somewhat too light compared to the surrounding text -- satisfies the OP's requirement. Should one need italic small caps in the (non-bold) running text, one can improve Boris' solution by not only switching to the italic capitals of a smaller font, but also switching to bold.

\documentclass{article}

\usepackage[osf]{mathpazo}

\usepackage{relsize}

\newcommand*{\textfakeitsc}[1]{\textit{\textbf{\MakeUppercase{\smaller[2]#1}}}}

\begin{document}

\Large

Some text \textsc{in small caps \textfakeitsc{and faked italic ones}}.

\end{document}