Saiga's take a slightly different magazine than regular Ak mags, to get around 922 since stock Saiga mags are all low cap.
The front of the magwell in a Saiga is different than a regular Ak. However if the latch on the back of an Ak mag is filed down, and a notch on the front of the mag is filed out it will fit in the magwell, although it may not seat or feed properly.
IMHO- surefire makes an excellent highcap saiga specialty mag that properly fits, quickly, and has a last round hold open, although the bolt slams shut when the mag is removed. I have not had to modify the surefire mags at all. I keep these loaded for HD and use them as my second mag when training so I can do a quick reload if necessary.
Mastermolder/Thermold mags are second best. They feed just as reliably as Surefires but need some modification. I got mine second hand, so I don't know exactly how they were modified but it looks like the back notch was filed down. Originally, I thought thermold made these specifically for saigas, and the back was filed down to tweak it a bit as is often necessary for Ak's. However the Thermold engineer at the Shot show said they have never made saiga mags.
Promags, even the saiga specific ones are the worst of the ones I have tried. The front notch is just not right and it can take a few concentrated attempts to get the mag in there. Also there is something wrong with the follower. The last round can jam. - I would stay away from these.
My Siaga has been converted to 922 compliance, however, I've never installed a bullet guide. I have enough mags that work reliably (thousands of rounds with no problems) that it is not a priority for me.