Yes, I did a fit check on it last night and it fits perfectly. The Eshopps RS-75 sump is 18" long x 10" wide x 16" high. The cabinet on the 250 is about 19 inches deep so it has to go in that way, but it will still work fine. When I did the fit check I put the "vacuum" hose to the sump in the back and it left the back cutout of the stand perfectly. I'll post some pictures soon.
Lifereef.com sells overflows and modified covers for RSM's.
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No, IMHO you still need the overflow box in the 250. You can use the accessory port 5/8" connections for other things (chiller) but the overflow box is your safety switch between the DT and sump preventing flooding your floor. The Lifereef RSM Nano Prefilter is not only the best overflow you can get, it was designed for the RSM tank and the deeper inter cup needs of the RSM back chambers. Good luck!
Understanding Sumps