I seem to recall there being something negative that happens to you if you get charged in the flank or rear by a fear/terror causing unit. Is it an automatic panic test by chance? I can't seem to find it in the BRB
Nothing special for flank or rear charges, you are thinking of an older edition. In 7th, being charged by a fear or terror causer triggers a fear or terror test respectively. Even if they charge your front.
Not that I know of. From any side you need a fear/terror check if charged by them, the result of which depends on who outnumbers who.