It is up to the asset issuer how many digits after decimals point to allow (max 8). Internally, all amounts and prices are of course handled as integers (java longs), in the smallest possible unit. That "decimals" parameter is for convenience only when displaying assets like US dollar and Bitcoin in the UI, so that USD can be defined with decimals=2, and BTC with decimals=8. But internally, those will be handled in cents and satoshis.

So if you want to express quantities down to one millionth of a stake, it is certainly possible, just define the asset with decimals=6. Or, you can indeed define one asset unit equal to one NEM, then everybody who has 1 stake gets to have 1,000,000 asset units.

The only limitation is, the cost of a bid order (the amount you can trade in one transaction) needs to fit in a java long. But as the total available amount of NXT, expressed in NQT (our equivalent of satoshi), fits, unless NEM gets really really expensive, this should not be a problem.

Try it on our testnet first if you find all that fractional amounts definitions confusing.