Trading Eidolon Base Forms Free Evolutions

[Luna_Silvertear]

The title says it all. I wish to create an eidolon with the upper body of a woman and lower body of a serpent, similar to a yuan-ti abomination. Is it possible to switch out the free evolutions of one of the base forms, Either biped or serpentine, depending on which would help me accomplish what I wish or as close as possible to it at 1st level. Is this rules legal?

EDIT: I'd like to trade out the biped's Limbs (legs) for Tail and tail slap. This would accomplish what I want with the point value being even, leaving me with the three starting points (with an additional two if I take Extra Evolution pool feat).

[Weirdo]

It seems like a fair deviation from the rules and I'd allow it if I were the GM, but within the rules I think you'd need to either add a tail and tail slap to a biped, and ignore the legs, or to add limbs (arms) to a serpentine eidolon. Either way you'd have one evolution point left.