Physician associates are university graduates who have then gone on to do a 2 year postgraduate diploma in physician associate studies - learning basic medical skills and knowledge. What this means in practice (I think) is that a PA can work alongside junior doctors and nurse practitioners, taking histories, examining, doing clinical skills such as cannulas. They are not allowed to prescribe.
I have worked with student PAs but never with qualified PAs. I don't think I would fit them into a hierarchy as you are suggesting - ie I don't think they are above or below FY1s. They do a lot of the same things, but they're a bit different. I would see them as just another member of the team to work alongside, to help get our collective job done - just the same as I would count nurses, physios, pharmacists… Ask one of them to explain their role to you - if you ask in the right way, you wouldn't be seen as rude. 'Are you junior or senior to FY1s" is probably not the right way - but "tell me about your role and responsibilities" shows that you're interested and respectful.
As to your other question, about input from nurses / pharmacists - I have generally found this to be very helpful. As a brand new FY1, most of us were very junior and our nurses covered our backs and could have no doubt done our job for us - but they helped us, taught us, looked out for us, were patient with us when they could have done it 10x faster. They just get us licked into shape and then 4 months is up, off we go, and in come another brand new lot, start all over again. Same with pharmacists spotting the drug errors etc. As far as prescribing is concerned, of course you need to make sure that you're happy with what you're prescribing, as at the end of the day you have responsibility for the prescription you're writing. However in general terms, nurses and pharmacists know what they're talking about. My suggestion: thank them for their advice. If you want to check it in the BNF / with a colleague, that's fine. If they're waving a drug chart at you and want it right now, tell them in a friendly polite way that you're sure they're probably right but you're quite new to this and would just like to check. Lots of thanks and friendliness always diffuses a situation well, even if underneath you're feeling a bit irritated. Be careful that you're not coming across as a little bit aloof and irritated, even if that's not what you were intending, as that might be what's in return causing attitudes that you're perceiving as rude from other people. These folk can teach you a lot. They are not always right - but they are a lot of the time. And you need them watching your back.