GNrueil,     As seen here, the disk types can't be mixed within the same disk group.   To answer your other question, the VM's performance is based on resource allocation, the I/O performance difference is not an issue for ESXi or most applications. Frankly, so long as you don't demand more than what it can provide, any storage would work fine.   Let me know if this helps.        
Hello,  I have a storage with a set of SSD and SAS disks.  I would like to know : - if they can be mixed together in same pool as single to get better performance ?  or the only possibility is to separate them in different pools , ie A and B ?  - In the scenario we need to separate SSD set and SAS set, and attached to Esxi, as Datastores, the VMs running on SSD datastore will be faster than VMs running on SAS datastore. Is there a way or any techniques to overcome this  ?  Cheers,   
ME4024 - Hybrid Disks Setup