In order to avoid being attacked by Chryssalids, it would be nice to stay more than one action turn away from them, so that they cannot move up to my soldiers and attack. What is the movement rate of a Chryssalid, so that I know how many squares away I should place my soldiers?
The Chryssalid run distance in the ini file is 20. This translates into 13 squares of straight line travel for a half-move. See the detailed answer to the "What do distance units mean?" question.
A basic soldier moves 15 squares in a straight line during a full move. Given a Chryssalid standing next to a soldier and a clear straight run in the opposite direction, the soldier should be able to full move out of range of the alien's half move, making the alien unable to melee this turn. The alien would have to full move and attack, which it can't do.
Also note: Chryssalids ignore cover. They do not seek it. They do not use it. Their attacks ignore it. Your soldiers can better support each other by also ignoring cover to get closer together.