I dunno, just skimming the law:

tl;dr version: Same considerations as a private citizen with a license to carry, just with some exemptions on where they can go if they're on duty and responding to an emergancy. IANAL disclaimer here.

Long version:

https://legiscan.com/TX/bill/HB435/2017

1) Grants limited protections against an emergency services personnel in the event that they have enter a 'Gun Free Zone' while carrying a weapon while on duty and responding to an emergency (i.e. A medical emergency in a school). Note that for this to apply, you must already hold a valid license to carry.

2) The government is not liable if the responder fires his weapon, as doing so is considered outside the scope of their duties. You fire in self defense, you're doing so with the same considerations as if you were off duty as a private citizen.

I think the key part hinges on departmental policy regarding this. If they make it mandatory to carry, then yes that will likely cause problems. If they prohibit the volunteers, then no big deal, nothing changes.

If they make it a discretionary choice left to the individual volunteer, with the understanding that they do with the same liabilities as a private citizen (with the noted exceptions), then I think it can be a positive thing. Those that feel a firearm would hinder them will simply choose not to, those that feel that it is a necessity have the option to.