I cant comment on 250, but a big group of cpd's would be awesome. I just got 6 in my Mini M. I really like the fish. From what i've seen over the last few weeks, the six I have are somewhat a group but spend time solo when not in fear. The tank I bought mine from had 50 to 70 in there. They acted more as a group then my six. I don't know if i've ever seen that many together. Go for it.
I also think 250 Boraras Briggitae would be cool.