You don't need followers unless you state dictates that your mags cannot function properly with any other caliber. Such as your state requires your magazine to only hold 10 rounds of .458 SOCOM and cannot hold anything else. In that case, the sled follower prevents the magazine from operating properly with 5.56 ammo because it will auto dump the last two or three rounds in the magazine all at once and jam up the rifle. But if you live in a free state, don't waste your money on the followers. Just get any quality 5.56 magazine but P-Mags are still a no go with the .458 SOCOM with many of the bullets used in it. P-Mags will work with some bullets, such as JHP because you don't seat to the full 2.250 inches, but with spitzers like the 250-300 gr. Monoflex and TTSX bullets, the rib in the front of the magazine screws things up when seated to full magazine length of 2.250 inch.
The mags I use almost exclusively are Lance AWMs, 20R and 30R, but I also have had no issues with just about any 20-30R standard aluminum magazine. If it will work for the 5.56, it will work with the .458 if your barrel extension is not jacked up on the barrel from the maker trying to reinvent the wheel. If you have a quality barrel, like one from TROMIX, TJ, SBR, SSK, or RRA, it will work with completely unmodified 5.56 magazines. The only thing you might consider doing to any magazine is cutting the half moon in the front of the mag so unloading unfired rounds is easier or if you are shooting very large meplat bullets. You don't even need the half-moon cut if shooting spitzers or RN bullets.