the FS rounds need more spring pressure to load them properly, nothing wrong with that. this applies to the function of any linear mag in general... you could load .68 cal paint in with the FSR spring, but it could be enough pressure to break some more fragile paint in the mag, which would suck... which is why they include a lighter spring.
edit: that looks quite easy to change from mag to loader feed... I like it. that being said, I'd be using this marker mag-feed only... I think it would be very quick to reload these mags on the field due to the lock-load design (I wear a stock-class harness all the time anyways).