I have a cluster of ESXi Servers where servers with 10Gb uplinks are mixed with servers with 1Gb uplinks. Are there any issues with this in deploying VSAN on every host in the cluster?
Not from what I know. Just make sure that datalink throughput will fit the IOPS requirements.
Also, here are few Best Practices, might be helpful:
For a hybrid config (SDD cache with traditional drives for Capacity) you'll need a dedicated 1GbE uplink for the VMkernel interface that has VSAN Traffic Enabled.
For an all-flash config, you'll need at least a 10GbE connection (overall).
Remember that Virtual SAN 6 includes the ability to run Distributed Switches (vDS) and you can use Network IO Control if you'd like.
Also, all hosts (contributing disk groups or not) will need to meet this requirement.
I think "it depends" on the workload.
We have VSAN hybrid cluster where each host has a dedicated 10GE NIC for VSAN (we changed this from the initial design where we wanted to have also VM and management traffic as a standby config for this NIC).
During our load testing we saw spikes where the 10GE VSAN link is nearly saturated.
Driving VSAN with only 1GE NICs from my point of view would only be possible for small workload szenarios.