i think 1.000 is a figure of speech, sure the chapter is amde up of 10 companies of roughly a hundred men each but considering there are always brothers in training scouts and marines dying keeping the companies at exactly 100 men is impossible and thus keeping the chapter at 1.000 also is.
Consider this knowing that they'll suffer losses soon enough do you really think that the chapters ignore a worthy recruit just because it may take them above 1.000? What the astral claws did was keeping their geneseed for themselves to keep making more marines, that made it quite obvious what they were up to. a chapter that is well beyond 1.00 may have a problem because of it but a hundred marines over or under the limit no one will care about. they die in combat soon enough.
and as for the BT the thing the imperium does when a chapter goes over full strength is order them to do a crusade, and that's kind of meaningless when it comes to the BT.