VCF is an advancement to Juniper's VC technology. When we say VCF, it is an Ethernet fabric, and whole fabric acts as a single switch - one IP to manage the entire fabric.
Now, consider your VCF as one chassis, and you can connect other devices as if you are connecting those to a single device.
All good as long as you are not going to make new switch(es) part of the fabric, in that case, you have the restrictions, i.e. what specific models can join the fabric.