A.P.
I work for an auto insurance company in a claims department. First rule (and this is for everyone), there is no such thing as "full coverage." Every coverage (in Texas) except Bodily Injury/Property Damage (BI/PD) is optional, so the definition of "full coverage" changes from person to person.
Since you are at fault for the accident, anyone injured or inconvenienced by the accident can file a BI claim against your policy. The only limits that matter there is your BI/PD coverage. All of those claims have to be processed using those limits. Based off of the info they currently have, your insurance company is telling you that your limits are not enough. Well, read carefully, because they may be telling you that it MAY not be enough. Regardless of what happened at the scene, people do have a right to come back and pursue a claim. Many claims are processed where people don't feel the effects until days after. And even if they are not hurt, they can get a small settlement just for lost time, wages, or inconvenience. Again, read the letter closely. The insurance company may also just be letting you know that someone is represented by an attorney so the likelihood of a lawsuit has increased and what you should do if you have been served. The exact language is crucial here, so read carefully and call the adjuster if you have questions. You should be aware that your Property Damage limit is separate, and with five cars involved, it is almost a guarantee that you do not have enough limits (depending on the age of the vehicles and amount of damage, the minimum 25k would not be sufficient). Bodily Injury is a separate issue.
Since there is a high percentage of individuals in Texas that break the law and drive without insurance, many people have Uninsured and Underinsured Motorist coverage. Most people will use this coverage under their own policy before coming after the at-fault driver personally. However, they can still come after you if your limits aren't adequate, for injury and/or property damage.
Again, confirm if these claims WILL exceed or MAY exceed your limits, and go from there. I suspect that it is the standard letter for the BI, but not for the PD. I wouldn't actually go hire an attorney until you have been served, but I would go talk to one or have some contacts now, just in case.
**EDIT**
The insurance company is only required to represent you if they are denying the claim or if they can't come to an agreement on the settlement amount. Essentially, if it is the insurance company who is standing firm or did something wrong, they are required to hire counsel for you. If all the claims exceed your limits and the insurance company processed the claim in good faith, they CAN come after you, including filing a lawsuit. The insurance company is not required to represent you at that point. Also, if person does use their UIM, that insurance company can come after you (it's call subrogation). They will look up your assets first and if they think they can't get much out of you, they won't pursue. But they will try.
HTH! Let me know if you have any more questions.