Re: warm ups
The "don't train the same muscle groups on consecutive days" theory derives from the body-building methodology which gets de-constructed a lot around here. Spend some time on the boards, watching the videos and you'll get a GREAT understanding of the CF methodology and how it contrasts with body-building.
I train 2-10 CFers out of my garage gym (lvl 1 cert) 5-6 days a week; my peeps come from widely ranging fitness backgrounds and I have them all doing the CFWU every workout. I scale it to their respective fitness level, those who can do pull-ups do pull-ups, those who can't use bands to assist them or do jumping pull-ups. If a CFer can't do a push-up let alone a dip, I start them on knee push-ups, graduate them to real push-ups then to dips with a band to true dips. You get the idea.
By gradually increasing the demands placed on their bodies, they adapt to the stress; sometimes astonishingly quickly.
I hope this helps. Enjoy CrossFit it can be life-changing.