In the BSCI (page 7-94), I read concerning the option summary-only : ?when the more specific routes are supressed, they are still present in the BGP table of the router doing the aggregation. However, because the routes are marked as suppressed, they are never advertised to any other router.?
Is that means that the suppressed routes are not advertised either inside the same AS (to a iBGP peer) ?
With your requirements I would omit the "summary-only" and use a route-map/prefix-list between eBGP peers to filter more specific networks, but allow the aggregate.