Will we be able to buy western size clothes and shoes? This greatly depends on what western sizes you are, I'm afraid! I'm a UK size 12 and I struggle to find clothes that fit me unless I buy over-sized t-shirts! You can buy online, I usually use a UK shop and have things delivered here. I'm also quite busty, be prepared to not find a bra that fits if you are over a B-C cup! Bring back-ups with you! I sometimes struggle to find shoes being a UK6. I usually have to buy men's trainers.
Will we be able to get prescription medicines? Yes and no. Again, it will depend what you're looking for. Most pharmacies will sell western anti-biotics, but self-prescribing is not advisable. Chinese doctors rarely give you western style medicine. I dislocated my shoulder last year and instead of a medical sling, the doctor advised me to use my winter scarf as it was cheaper! The biggest problem you'll have is that most medicines are (of course) written in Chinese, so unless you now what you're looking for, it can be difficult to find what you want. I once had to mime sniffing a plant and sneezing to get hay-fever tablets!
Will we be able to manage on a salary of 6000 rmb each per month? It can vary depending on city, but as you have accommodation provided I'd say this is easy enough to do. For 6000rmb and accomodation provided I'd hazzard a guess you're going to an EF? Be careful! As a 3 year EF China veteran, you need to make sure the EF you go to doesn't take advantage! 6000rmb is quite a low wage, lower than most other schools. But perfectly liveable on, so long as you aren't in a major city like Shanghai or Beijing!