Alteburger - The precise requirement in relation to rates of climb and descent when flying in accordance with an ATC clearance is that pilots should inform the controller if they anticipate that their rate of climb or descent will be less than 500 fpm, or if at any time their vertical speed is actually less than 500 fpm.
This applies only when above the Transition Altitude. Below the TA there is no obligation to tell ATC about a low descent rate being carried out for the purpose of complying with Continuous Desent Approach Procedures or indeed any other reason.
To summarise, there is no prohibition as such on rates of climb or descent of less than 500 fpm when necessary for your own operational reasons as long as when above the TA you tell ATC.
In the Gulfstream/B747 airprox, the fact that the crew of the B747 did not inform ATC that they had reduced their rate of descent in order to slow down was only one of the causal factors.