I'm not able to make small caps work with any font in XeLaTeX. Here is output illustrating the issue for Charis SIL, Garuda, and Times New Roman, respectively. Pasted below are a MWE and version and package information.
Minimal working example:
\listfiles
\documentclass{article}
\usepackage{fontspec,xunicode,xltxtra}
\def\sample{The quick brown fox jumped.}
\begin{document}\Large
\fontspec{Charis SIL}
Normal\hfil\sample\par
\textsc{Small Caps\hfil\sample}\par
\textbf{Boldface\hfil\sample}\par
\textit{Italics\hfil\sample}\par\medskip
\hfil\rule{6cm}{5mm}\hfil\par
\fontspec{Garuda}
Normal\hfil\sample\par
\textsc{Small Caps\hfil\sample}\par
\textbf{Boldface\hfil\sample}\par
\textit{Italics\hfil\sample}\par\medskip
\hfil\rule{6cm}{5mm}\hfil\par
\fontspec{Times New Roman}
Normal\hfil\sample\par
\textsc{Small Caps\hfil\sample}\par
\textbf{Boldface\hfil\sample}\par
\textit{Italics\hfil\sample}\par
\end{document}
\listfiles
output:
*File List*
article.cls 2007/10/19 v1.4h Standard LaTeX document class
size10.clo 2007/10/19 v1.4h Standard LaTeX file (size option)
fontspec.sty 2013/05/20 v2.3c Font selection for XeLaTeX and LuaLaTeX
expl3.sty 2013/07/28 v4582 L3 Experimental code bundle wrapper
l3names.sty 2012/12/07 v4346 L3 Namespace for primitives
l3bootstrap.sty 2013/07/28 v4581 L3 Experimental bootstrap code
etex.sty 1998/03/26 v2.0 eTeX basic definition package (PEB)
l3basics.sty 2013/07/28 v4581 L3 Basic definitions
l3expan.sty 2013/07/24 v4565 L3 Argument expansion
l3tl.sty 2013/07/28 v4581 L3 Token lists
l3seq.sty 2013/07/28 v4581 L3 Sequences and stacks
l3int.sty 2013/07/28 v4581 L3 Integers
l3quark.sty 2013/07/21 v4564 L3 Quarks
l3prg.sty 2013/07/28 v4581 L3 Control structures
l3clist.sty 2013/07/28 v4581 L3 Comma separated lists
l3token.sty 2013/07/28 v4581 L3 Experimental token manipulation
l3prop.sty 2013/07/28 v4581 L3 Property lists
l3msg.sty 2013/07/28 v4581 L3 Messages
l3file.sty 2013/07/28 v4581 L3 File and I/O operations
l3skip.sty 2013/07/28 v4581 L3 Dimensions and skips
l3keys.sty 2013/07/28 v4581 L3 Experimental key-value interfaces
l3fp.sty 2013/07/09 v4521 L3 Floating points
l3box.sty 2013/07/28 v4581 L3 Experimental boxes
l3coffins.sty 2012/09/09 v4212 L3 Coffin code layer
l3color.sty 2012/08/29 v4156 L3 Experimental color support
l3luatex.sty 2013/07/28 v4581 L3 Experimental LuaTeX-specific functions
l3candidates.sty 2013/07/24 v4576 L3 Experimental additions to l3kernel
ifpdf.sty 2011/01/30 v2.3 Provides the ifpdf switch (HO)
xparse.sty 2013/07/28 v4582 L3 Experimental document command parser
fontspec-patches.sty 2013/05/20 v2.3c Font selection for XeLaTeX and LuaLaTe
X
fixltx2e.sty 2006/09/13 v1.1m fixes to LaTeX
fontspec-xetex.sty 2013/05/20 v2.3c Font selection for XeLaTeX and LuaLaTeX
fontenc.sty
eu1enc.def 2010/05/27 v0.1h Experimental Unicode font encodings
eu1lmr.fd 2009/10/30 v1.6 Font defs for Latin Modern
xunicode.sty 2011/09/09 v0.981 provides access to latin accents and many oth
er characters in Unicode lower plane
eu1lmss.fd 2009/10/30 v1.6 Font defs for Latin Modern
graphicx.sty 1999/02/16 v1.0f Enhanced LaTeX Graphics (DPC,SPQR)
keyval.sty 1999/03/16 v1.13 key=value parser (DPC)
graphics.sty 2009/02/05 v1.0o Standard LaTeX Graphics (DPC,SPQR)
trig.sty 1999/03/16 v1.09 sin cos tan (DPC)
graphics.cfg 2010/04/23 v1.9 graphics configuration of TeX Live
xetex.def 2013/04/29 v0.96 LaTeX color/graphics driver for XeTeX (RRM/JK)
fontspec.cfg
xltxtra.sty 2010/09/20 v0.5e Improvements for the "XeLaTeX" format
ifluatex.sty 2010/03/01 v1.3 Provides the ifluatex switch (HO)
ifxetex.sty 2010/09/12 v0.6 Provides ifxetex conditional
realscripts.sty 2013/03/18 v0.3c Access OpenType subscripts and superscripts
metalogo.sty 2010/05/29 v0.12 Extended TeX logo macros
t3cmr.fd 2001/12/31 TIPA font definitions
Finally, version info for my OS and TeX distribution:
XeTeX 3.1415926-2.5-0.9999.3-2013060708 (TeX Live 2013)
kpathsea version 6.1.1
Copyright 2013 SIL International and Jonathan Kew.
There is NO warranty. Redistribution of this software is
covered by the terms of both the XeTeX copyright and
the Lesser GNU General Public License.
For more information about these matters, see the file
named COPYING and the XeTeX source.
Primary author of XeTeX: Jonathan Kew.
Compiled with ICU version 51.1; using 51.1
Compiled with zlib version 1.2.7; using 1.2.7
Compiled with FreeType2 version 2.4.11; using 2.4.11
Compiled with Graphite2 version 1.2.1; using 1.2.1
Compiled with HarfBuzz version 0.9.15; using 0.9.15
Compiled with fontconfig version 2.10.93; using 2.8.0
Compiled with libpng version 1.5.16; using 1.5.16
Compiled with poppler version 0.22.2
LSB Version: core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch:core-4.0-amd64:core-4.0-noarch
Distributor ID: Ubuntu
Description: Ubuntu 11.10
Release: 11.10
Codename: oneiric