The Borg are rather choosy about what race they want for an assimilation model. Not too big as they might jostle people in the hall. A head, arms and legs that are efficient to work with. Species 8472 are big ugly insects and they might be incapatable with their nanoprobes since they have twice as much dna structure (which accounts for their superior adaptability)as well as being poly-fluid based. They could be based on hydro-carbons or gas.