The answer is you can but this is strongly discouraged for performance and network throughput reasons.
The Cenelec enabled devices will be swamped by traffic from the Cenelec disabled devices as the enabled devices will listen for the additional time of 4ms for a tone at 135.5Khz as per the Cenelec specification. The data transmitted by the disabled devices looks like that tone as far as the enabled devices are concerned and thus will prevent transmissions from these devices. This is particularly so if the disabled devices generate a lot of traffic and make use of priority messages as priority messages occupy the powerline bandwidth ahead of any other normal non priority messages. This applies mostly to LNS managed or other peer to peer configured networks
Note that smart server outgoing messages cannot use priority but the channel properties can be configured in Cenelec or non Cenelec mode.