Hello Evan,
As always it is 'best practice' to use homogeneous hardware on each node if possible, but Yes it is supported to use mixed drive models in the same disk groups provided said drives are on the vSAN HCL. At the inter-disk-group level the main concern of using mixed disk models in the same disk-group would be the controllers driver/firmware not playing as well with one model of driver than the other, but if is same vendor and near-enough same family of model you should have no issues with this.
Realistically a capacity-tier drive is very unlikely to be saturated enough to use the full 6Gbps (let alone 12Gbps) so unlikely to perform better (and anyway even if it did, this would be negated by 'waiting' on the other slower disks)
Also bear in mind, that even if you just need say 1TB more space right now, you will need to add the same amount of space to each node (not necessarily each disk group if you have more than 1 disk-group per host, but again - having homogeneous disk-groups is best practice).
Bob
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