Yes you may only use your own writings from another website for a hub, just note that Hubpages may detect duplicate content and will flag your hub with a duplicate flag.
A duplicate flag does not necessarily mean you are plagarising, and your hubs can stay published and as it is for people to view.
However it is unlikely that your hub will be ranked highly on Google, and you cannot post links in your hub.
About your next question, too short a hub will flag as shallow content or something to that extend.
If you want it safe, about 400 words should be decent for a hub.