Legal Question in Wills and Trusts in Texas

Any pro se representation is dangerous. When it involves an actual dispute and the other side has an attorney, it's foolish. That said, a pro se party must abide by each and every rule that any attorney would. The sources of the rules that govern attorney conduct are numerous. Since the pro se party wants to be their own attorney, the Court will treat them as if they are an attorney, and will hold the pro se party to standards of conduct and procedure that they simply cannot and do not know.

The above answer is correct, but to answer your specific questions:

You can speak to the other attorney as much as you want (if he or she agrees to do so). You may not discuss the case with the other party without the attorney's approval.

Most, but not all, of the rules are found in the Texas Rules of Civil Procedure, the Texas Rules of Evidence, and the Texas Rules of Appellate Procedure. Yes, some of the appellate rules affect the trial court, also.

You MAY provide any information that you want, but you MUST respond to or properly object to formal discovery.

As Brian said, it would be foolhardy to do this yourself if the amount of money in controversy is significant to you. I can think you dozens of things that you might do or fail to do that would hurt your case. For example, if you do not properly authenticate your documents, then the court will never see them. Likewise, if you do not know how to object if the other side fails to authenticate their documents, then the court will see them even if they would otherwise be inadmissible. If you fail to object properly to inadmissible testimony (or anything else), then you cannot later appeal that issue. If you do not properly respond to discovery, then the court may award the other side their attorney fees for getting you to or not allow you to present all of your evidence. You not only need to know all of these rules, but in trial, you must know them well enough to immediately object or the objection is waived. Am I trying to scare you? Yes. I would hate so see you hurt just because you think that you can do this yourself. I have seen it many times before.