I've never heard of removing the ignitor for halides. A few years back there was an "HQI" 400w MH ballast which was just a SonAgro ballast (430w) for sodiums. Same everything, just run a halide instead of a sodium.
In fact it might work better than a 400w probe start ballast since most halide lamps in the hobby are meant for pulse start. Probe start ballasts do work (mostly) but are hard on the lamps (shorter lifespans usually).
400w is an awful lot of light for a 55g. You will have major heat issues to content with. Not saying it can't be done but it is definitely overkill.
Unfortunately no you cannot mix and match wattages with lamps and ballasts.
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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