If you can't match the insert to the pipe then match it to the fitting, that way you're at least 50% right 
As suggested, as long as the insert isn't super loose then it'll be fine. After al., the insert is there as a support for the pipe to stop it deforming and the grab ring in the fitting from popping off. That's one reason they pressure test push fit to such a high pressure, as it helps bed the grab rings in properly.
When using push fit, once it 'pops' all the way home, I pull it back on it a little, it just helps the teeth bite before water pressure is run through the fitting.
As suggested, as long as the insert isn't super loose then it'll be fine. After al., the insert is there as a support for the pipe to stop it deforming and the grab ring in the fitting from popping off. That's one reason they pressure test push fit to such a high pressure, as it helps bed the grab rings in properly.
When using push fit, once it 'pops' all the way home, I pull it back on it a little, it just helps the teeth bite before water pressure is run through the fitting.
