Yes, you can, in exactly that way. The OLD lists them as separate entries from the normal sense of each word, and calls them "indeclinable substantive adverb".
Cicero: nos ama vel, si id nimis est, dilige.
Love us; or, if that is too much, like us.
Makes sense...
satis in this sense can also be shortened to sat.
Hadn't seen that before
That would be "nonnumquam", btw, but I'm sure that was just a typo. My intuition is telling me the word-order could be slightly altered: "nonnumquam nimis non est satis". "...non sat est" sounds good as well.
A typo indeed I haven't got much in the way of intuation yet...need to read some more. I'm beginning (I think) to get a feel for when ergo is appropriate, and when igitur.
~netweaver~