I feel like everyone who ever wrote a guide there for someone who needed help has felt that way at least once or twice. The negative experience you are describing is what most people experience when they write advice that they are unsure of. I try to completely dodge trades where I am not 100% sure (mostly with pets I don't trade that often like BW's or massive bulk trades of common/uncommon pets. Exactly because of what you say, I can get scolded or disagreed with.
Disagreements are fine ofc, there are at least 3 or 4 different value systems that some older users are using and sadly, people who are writing advice in FTT should only be using the new updated version of Horror's list and keep it simple to understand. It's fair to say that you are free to use any values you would like for your own pets when trading, just the advice for people in one thread should be solidified.
Best of advice I can give you when you try to help someone with their trade would be - if you are not 100% sure or even post a link sometimes to prove it, it's definitely better to write something like "I am not 100% sure, please get second opinion" on bottom of your advice. That way the person you are helping won't get that confused if someone suddenly disagree with you and then they have 2 different opinions not knowing what to pick
I wouldn't personally say it feels like "elitism" to me but usually giving advice to people on certain pets can get tricky due to demand. It's a factor that belongs to trading and has space in the advice you give (such as when you say spiders have very specific audience and therefore might have less demand for most people than dogs/cats ^^ - THIS is the correct approach and it does help people especially if you separate the advice by value + demand!)
About the old rares concept that you were confused about - you are not the only one there, don't worry about it ^^ It's due to a poll that was held and I believe maybe 200 or so participated. Because "old rare" used to be 09 pet and after the update and poll - it is not anymore. Now years 2010 and 2011 were merged together as same value and are classified as "old rares". Likely due to the amount of them in comparison to remaining 09 rares (there's really only few left). Personally I don't value them completely same in my own trades and I think giving someone 10x 2011 rare for 1 MA would not even be balanced and low. But this is not most shared opinion, it's just something I do. I either pick rares from those years 50/50, or only offer 2010...but still I don't do these trades very often.
What lately bothers me more is the amount of people who are spamming trade forum asking for trades and then proceed to decline all of the trades incoming without stating reasons. I am not only one who finds this little bit mean from the other person even if they did so because they are not sure of some value. It's always better to ask someone for help or negotiate with the person trading. So the amount of people who would actually edit the trade and try to spend some time on it or work something out is extremely low and therefore, people sometimes don't even send out trades...also some people just got "scammed" of their good pets too many times, which is why we should always ask and always help each other. I have been returning user at some point too and all of my pets were from 2008-2011. I lost majority of my pets right after return. Mostly because I wanted complete collection for the years missed and then things like trading 2009 VR for few new uncommons happen. It might be sad but it also made me learn from my mistakes and learn more about values and demand over the time. The fact that my trade fodder was filled with rats and ponies was my proof to it xD
I just hope we get more and more people who try to be nice to each other and communicate, rather then turning this site into "point and click" trade action. ^_^