Perhaps somebody more knowledegable can answer the question on flat wound strings.

They come in different weights, just like other guitar strings. The tension for a given weight (gauge?) is lower. I would think if you used slack key tuning on the guitar you might have trouble with the lower tension, although I used the AC111 which has the gauges:

.011, .014, .019, .027, .036, and .050

for bottleneck playing with the guitar tuned down, and the strings had enough tension. I have an old, lightly built guitar with the next size down from them and I have trouble using a slide with them--even a light touch pushed the strings into the frets; but the guitar has very low action. Your Regal probably has a much higher action, and you might not have that trouble.

You might not need flat-wound strings; because they are so smooth.

By the way. The inside of the label has the tensions for the different gauges. The package can be opened withoug destroying it, so you can go to a store and ask to look at that.

Murray