Soaking film overly long (or paper as well) will cause undue swell and softening and may cause the film or paper to be softer and more prone to scratching. In extreme cases and with lower quality products, the emulsion can float right off the support.
I buy my Ammonium Hypo 60% in bulk from the Formulary and mix my own.
PE
Two points:
1) PE really knows what he is posting about. He has been instrumental in the development of some of the best, most recently developed fixers;
2) be careful if your plan is to duplicate what PE does, because PE knows what PE knows, and most likely you don't!
Your Ilford Rapid Fixer is great. Follow the instructions carefully, and it will be very well suited to most commonly available films and paper.
If you transition to something like TF-5, and follow the directions carefully, than you will be using a fixer that PE co-developed, and you will be following directions he may have written, and he certainly approved.
And by the way, there is nothing that makes it necessary to include hardener with non-rapid fixers. They use sodium thiosulfate (formerly sodium
hyposulfate) and only tend to include hardener in commercially packaged mixtures because, historically, they included hardener, and haven't been changed.