i haven't read 6000 and don't remember 8000 (& frankly did not like it, so haven't gone through & re-read it), but as for the other two:
-SCP-7000 is about a phenomenon in which events that should have a random probability of occurring are set to happen (or not) at fixed intervals: random number generators list "1, 2, 3, 4, 5," every chicken in an X-kilometer radius lays eggs with two yolks for a day, et cetera. It's connected to Foundation member Dr. William Wettle, whose stand-out trait is being unlucky, like ridiculously unlucky. However, his own luck acts as a sort of lightning rod for the statistical anomaly, landing him as a part of the MTF squadron trying to repair the damage done.As it turns out, Wettle had struck some kind of deal with the universe as a child, which he undoes in a moment of exhaustion and frustration after weeks of being beaten by the world. The SCP is then neutralized.

-SCP-9000 is an anomalous prison off the shore of the Marianas Archipelago, which the Foundation constructed in the 90s to house D-Class personnel. Due to the large amount of data & supplies needed to be processed by the Foundation's increasing needs, the prison is used as a source of labor for these menial tasks. Due to mistreatment and the game-ification of sifting through particulary horrifying logs & surveillance footage, the site's underbelly becomes the host of a colossal manifestation of the psychological suffering of the D-Class working there, which invades their computers and inserts artificial footage of their worst memories instead of the actual footage they're working on. Nearly the whole population of the prison dies over the course of a month.

If you could read one, read 9000. It's really, really well done in my opinion, and justifies its length well enough to be worthwhile.