The retainer will not be the issue. It will be the ILS specific cap (which adds length) combined with a standard spring (which is longer than an ILS specific spring and also adds length - compressed).
If you are using all ILS specific parts (cap, spring, retainer) it should work just fine in an aftermarket MSH...because when assembled it is the same overall length as a standard set. You will run into issues if you start mixing parts as stated above.