My whole house is GE low-voltage, and I have been able to buy everything for it. In my relay racks, the RR-7ís go into the RR-5 holes. In my case, they seem to be the same size. Now some of my relays have sleeves on them, perhaps the older units. Others have spacers in the hole, itself. The sleeves only serve to keep the units from vibrating in the hole.
I would suggest you re-evaluate the fit of the RR-7ís. The RR-5's are only 15amp, and every used one I have ever bought was worse than what I was trying to replace. You may only need to modify the hole slightly, remove the sleeve, or the spacer.
The bigger thing to deal with is that the new relays are designed to simply plug and screw in as far as the wires are concerned. Makes it harder when the old system is hard-wired.