Starter, grower or all flock feed would all work for them.
However, at 20 weeks, it won't be long before they begin as days are getting longer faster every day. At my latitude, today is 2 minutes 10 seconds longer than yesterday. By the end of the month days will be 50 minutes longer than today. The pullets' pineal glands will detect that change and stimulate the gonads to produce the hormones resulting in sexual maturity and onset of lay.
What I'm getting at is that before long your pullets will begin laying. Another option at this age is to blend equal parts of layer and starter or any other feed. The reason is that there is very little difference between feeds when it comes to vitamins, minerals, subtle differences in fat, fiber and energy. But, the big difference is the calcium level in layer (. ± 4%) and the . ± 1% of all other poultry feeds. There is also a difference in protein levels that can be significant depending on the species and age of the intended recipient. (a gamebird maintenance feed can be as low as 12.5% protein to as high as 30% for a gamebird starter feed)
As a pullet approaches point of lay, they acquire the ability to store calcium in their medullary bones. Then when an egg is produced some of that calcium is immediately pulled from the bone and migrated to the uterus to make the shell and the bone calcium is replaced from the diet.
That makes crushed oyster shell valuable in that the large particles lodge for a bit in the upper digestive tract making that available when the egg enters the shell gland.
As many here do with a mixed flock is to feed an all flock or grower feed to all the birds and provide a separate container of oyster shell so the birds laying can acquire the needed calcium.
