I would say you definitely have one or the other, velvet or ich.
To cover both, I would use the cupramine treatment and a larger tank than a 10 gallon is definitely needed. what are the sizes of the fish?
Perhaps a LFS
would have a used 40 or 50 gallon tank you could pickup.
Hard to give you a tank size estimate without knowing the size of your fish.
I have treated a large stock in a 40 gallon before, it can be done, but I wouldn't ever suggest this treatment in your main for multiple reasons.
Also, if it is velvet, I would make a decision pretty fast. Once it hits the advanced stages, it can kill your fish very rapidly. I lost 3 fish to velvet in a 24 hour period. Not trying to scare you, just want to emphasize the need for action.