fuzzy i usually introduce yogurt 2nd after baby rice, so yeah i think its ok to start at 23 weeks- but i only give them yogurts for babies, petit filous and something else but i cant remember its name. it does say on the packaging that its suitable for weaning at 4months though. i waited till 12 months to give any other type of yogurts.
im not sure about cheese. i wouldnt because i would think its too salted, but thats only my gut reaction, not something i have read anywhere. i'll look into it. 
my thought process-but i know i am a bit militant- is that until they are 12 months, they have everything for babies, ie, baby cereal, pasta, sauces etc as i presume that there will be less salt in than the adult version, plus fruit and veg and a bit of meat that i have prepared, so i know what is in it.
i know i am quite strict though, cos j and i have never trief chocolate, crisps, cake or ice cream. they have had a biscuit but only a handful of times at playgroup. i told the woman not to give them one as i dont let them have them yet, but jacob managed to get hold of one when i turned my back to pick eva up. so i have relented and let them since. i take the attitude that they cant ask for sugary treats so why give them them? might as well fill them full of good stuff at this stage. they will more than make up for it as they get older, as my chocoaholic self can testify. 