try this in your fonts.conf
<match target="pattern">
<test qual="any" name="family">
<string>sans-serif</string>
</test>
<edit name="family" mode="append" binding="strong">
<string>Tahoma</string>
</edit>
</match>
Similarly for serif fonts:
<match target="pattern">
<test qual="any" name="family">
<string>serif</string>
</test>
<edit name="family" mode="append" binding="strong">
<string>Bitstream Charter</string>
</edit>
</match>
just replace any font u like in <string></string>