Can I write only "please vote" or "vote tripling" letters for TX-06?

That’s not possible for this campaign. Since we need both types of letters to be sent in roughly equal numbers, all of the letters are part of a single campaign, with the template assigned at random. The usual "I vote because ..." prompt is used on both templates, so there is no need to distinguish between them or adjust your handwritten message. By writing and sending both types of letters, you are making a valuable contribution to this experiment.