Hmm.. I'd say they should probably be on the same feeding routine as their mainland cousins, just smaller prey (similar in width to body girth). I think young retics typically eat every 5-7 days.
I think many of the people breeding SD/ D retics under-feed to keep them smaller longer. I know there are some people that say to do the same with mainlands. IMO, that is just cruelty when done to keep the snake smaller in size. Plus, many retics will push and ruin their faces when not fed properly.
I hope you can get them well-started and on a correct feeding schedule. I'd also be interested in asking the breeder why he is under-feeding. :/ I think with my CA motley male boa, he was underfed to use less food / $$ and be kept in a smaller tub size until sold.