Jalbert
It is perfectly acceptable. Doc 4444 states that "aircraft may be cleared to change cruising level at a specified time, place or rate" and the relevant phraseology is included. However, vertical separation plus the rate of climb/descent ("...or greater") condition would have to be established first and a limit placed on that condition ("...until passing FL...") before the second aircraft were to be allocated its level with ROC/ROD restriction ("...fpm or less").
Obviously cannot comment on the specific situation without further detail.
Regards
2 s