A question on tenses: what is acceptable, and what makes people point accusingly.

[scarylady]

Hiya all.

I have a question for you, if you'll indulge me.  I'm planning my next, long, chaptered fanfic, and as I'm making my initial forays into writing it I'm strongly feeling that, to get the style and feel right, I want a third-person, present-tense Prologue, and a third-person past-tense Rest of Fic.

Is this allowable?  Or is it sloppy?  I would prefer not to write the entire thing in present tense, but the Prologue is resisting all my efforts to re-write it into past tense.