I would think it is OK. Use gpart to crate slices of identical sizes on each drive and crate zfs out of those slices. Alternatively you could also theoretically create just a single dangerously dedicated slice on each drive and then use disklabel to create bsd partitons of same sizes to be included in the zfs mirror.
Or if you have no other use for the extra space on the larger SSD, just create a mirror out of the two SSDs. The larger one will be much less utilized, which is good for its endurance. Then in the future, if you get lucky and the smaller one dies, you can buy a new larger one, and end up with a 489G file system. Rinse lather and repeat, as newer hardware becomes available.