mule palms,bolivian coconut,very tropical

Chamaerops humilis (European fan palm--grows in clumps, drought tolerant)
Sabal texana (native Texas sabal)--very robust, nice-looking, and native to Texas, bigger than its Eastern cousin (Sabal carolina)
Sabal louisiana (quicker growing than most Sabal palms)
Trachycarpus forntunei and wagnerianus (grows better in the cooler months, slows down or stops in high heat)
Washingtonia filifera (hardier than W. robusta)

There are others but these are most commonly available in the US.

Im not sure why you are worried about 15 degrees?
depending on your properties exact location near the water down there you could likely grow anything that is commonly grown in orlando fl.

According to my map that area has only been anywhere near or below 15 degrees 1 time in the last 108 years. and that was 12 degrees in 1899. I dont know about you but, I dont plan for the once in a lifetime freeze otherwise I wouldnt have any palms growing.
the absolutely deadly (to florida and texas) freeze of 1989 sent the official temp to 18 in brownsville and based on the remaining palms of the area if you were nearer the water you stayed above probably 25-28 since there are some older less hardy palms still around.

id pin the min yearly low at abou 28 with water making that temp rise to at least 32-35....

I dont think this is too optomistic in looking at the palms growing down that way.

I'm going by the record lows for each date according to weather.com. It looks like there have been lows below 20 degrees every 15 years on average since 1880. I don't want to plant something and then have it frozen when it's 20 feet tall. It looks like the safest selections are pretty much the hardier sabals, chamaerops, and butias.

Drive around, there are probably plenty of mature (survived decades) palms around, just as I saw upriver in Laredo a couple years ago.

gardner, take a trip to weslaco, I can sell you a ton of palms that will handle your end of the valley!!