Agreed on all fronts, especially with player knowledge of the rules.
There's always the option that the characters can ask "can I make a nature/religion/arcana/etc check to see if I know anything about this creature?" and if they get a decent roll Adam can say "you know these are dangerous slimes that viciously corrode any armor or weaponry when touched, you've heard rumors that they duplicate when damaged/split". Even that little bit of info can ensure the party thinks differently about the encounter rather than tackling it guns blazing like most times.
Also you can move past/through enemies squares if it's up to +2/-2 sizes larger/smaller than you. They could have chopped the one blocking the exit down and just ran away, in fact they didn't even need to do that since I believe it was only a large creature so only 1 size larger.
I certainly don't think the encounter was as difficult as it played out, I've thrown a few black puddings at a group lower level than them and they did fine, it's just a matter of knowledge of rules, tactics and when to say "you know what, fuck this fight, it'll drain our resources and it's not worth it".