Fal Inch pattern mags are way more expensive and much harder to find than metric mags.
Stick with the metric mags for an stg58.
Unless you can find original Mill surplus mags the DSA mags can be hit or miss.
30 round 308 mags are heavy, they were for the Heavy barrel FALO for a LMG role, similar to BREN guns.
The FALOs and 30round mags fell out of favor in short order.
The DSA polymer mags are junk.
Don't shoot steel cased ammo in the FAL, they will have case head separations.
Have fun with the new FAL.