EDIT:
you can simply nest the expressions?, i.e.
{% for item in list if item.author == 'bob' %} {% if loop.index <= 5 %} do something {% endif %}{% endfor %}
to skip the first x elements you can
{% for category in categories[x:] %}
with all the expressions you can use for the regular lists
You can also use
{% for item in list[0:6] %}