I could be completely wrong, but I am sure I had non membership FF to begin, and it is really really basic. It will still tell you predicted fertile window and test date but I am sure you loose all the support and use of forums without the membership.

As far as your thermometer goes, I wouldn't think it matters what type of thermometer you use, but starting half way through a cycle may affect your temps as each thermometer is usually calibrated slightly different.

HTH