Question about Pure Bloods and House of Slytherin

[stacygalore]

I am trying to write a fic and was wondering about something.  Can the offspring and 
descendants of a squib be considered pure blood, if all said descendants are indeed wizards/
witches?  Is it possible for a squib to have wizard/witch offspring?

Is it a requirement for one to be a pure blood in order to be a student in the house of 
Slytherin?  

- Stacy