"HQI 250" is "ANSI M80".
"HQI 400" is "ANSI SonAgro" (actually a HPS ballast but works on MH).
Here are some PDF's from the advancetransformer.com webpage (Advance makes a lot of HID and other ballasts):
ANSI-M80:
http://advancetransformer.com/eCatal...2480932191.pdf
ANSI-SonAgro (there are 3 part #'s, not sure which one is which):
http://advancetransformer.com/eCatal...2480932192.pdf
http://advancetransformer.com/eCatal...2480932193.pdf
http://advancetransformer.com/eCatal...2480932194.pdf
You can look them (and others) up at this page here:
http://advancetransformer.com/ecatalog/hid_lookup.asp
Don't know about the halogen fixture fitting 250's vs. 400's. I actually thought they only held 150's but maybe I'm wrong?