Are you using TemplateColumn? If yes, you could put  validators statically
into EditItemTemplate with the TextBox and you wouldn't need to do dynamical
adding of validators in that case.

About maximum  length, by default such validator does not exist, but TextBox
has MaxLength property that you can use to set the maximum allowed length.
Although I am not sure how it behaves with other browsers than IE. It's not
hard to create custom validation control or CustomValidator to validate the
input length either.