This isn't entirely accurate anymore.
The QFX5120-48Y can't have SFP-T's inserted vertically when using older SFP-T variations due to physical restrictions. With newer variations, however, you can populate all 48 ports. Juniper no longer ships the type that has the restriction, so no special part number is needed. Obviously, I cannot speak to what you may have installed now or any third party optics.
With that in mind there is no reason you couldn't populate all 48 ports with either SFP+ or SFP-T.