Since each disk is independent in Unraid, the speed of those disks would only matter if you were reading or writing them, or possibly if you were using "Turbo Write" where all disks are read in order to update parity. I would say don't worry about it.
But, I always recommend not installing more disks than you need for capacity. Each additional disk requires an additional port, additional power, at some point more Unraid license, and most importantly, each additional disk is an additional point of failure. Don't install old disks just because you have them.