It really depends on what you want to do. The system itself is just for serving files and such right? If you want to use different sized hard drives and don't mind spending a little money on an OS, Unraid is the perfect option for you. The only bummer about unraid now is that they switched their licensing up a little bit and with the Starter version you only get updates for a year (vs lifetime in the past). Having said that, it's usually not an issue if you're not using other options in Unraid such as virtual machines etc. The starter version is $49 and would let you use both SSDs as scratch disks since unraid is installed on a USB drive.
Otherwise you can still use TrueNAS with your drives, but you would be limited by the smallest hard drive (eg if you had two 18TB drives and a 10TB, you would only effectively have 3x10TB drives for the pool).