Carla,
The recommendation of mixing the starter ration with oats is from Storeys Guide. I have not personally used the feeding regimen but will be doing it on this batch of pullets.
In general, delaying reproductive onset (puberty) will increase the longevity of reproductive life and increase...
More than likely, the oats are of the low protein variety. I don't think you can get them at the grocery store. The personnel at the feed store should be able to tell you what the protein level is...low protein varieties are 8 - 11%. I wouldn't start the mix 50/50 right at 8 weeks of age but...
You got the information correct. The oats will delay laying (via protein deprivation) and allow the pullets to acheive a larger body size before onset of laying. This will normally increase longevity and decrease the chances of prolapse...too big egg exiting too small vent.