So, I put a new chain on last week and then went out for a big ride - I figured it wouldn't be a big deal to leave a few extra links in as the derailleur tension should take care of it (I was tired at the time... whatever)

I couldn't get out of the saddle or put basically ANY pressure on the pedals in the smaller rear cogs w/out the chain slipping all over the place. I was like, "Piss... I guess I should pull those extra links out." and I did. But now it is too tight and won't go up into the bigger cogs. PLUS it is still slipping over the smaller cogs.

Does that mean it isn't the chain tension? Do I need a new rear cassette? I've got an older bike (80s Colnago) and someone in a bike shop said something about needing a freewheel instead of just a cassette? Honestly, I'm at a bit of a loss on how to go about fixing it all.

Thoughts?