"If my fiance comes to the U.S. "with a visitor's visa first to visit me and my family prior to us applying for the K-1 visa, will that pose a red flag?"
Absolutely not, it's highly recommended, in fact, meeting in person is even a requirement to even be eligible for a fiance visa. You must physically meet in person at least once within the 2 years prior to filing. It's also recommended, although not required, that you meet multiple times.
Now, if your fiance comes to the U.S. on a B1/B2 tourist visa, or ESTA (visa waiver) as a non-immigrant, with the intent to immigrate and stay permanently, yes, this could pose a major red flag. That's not to say you can't get approved for such adjustments of status, as many of them are still approved. But in the case of visiting on a tourist visa before applying for a K1 fiance visa, this isn't a problem in and of itself. In fact, the bigger problem you'll likely have is getting approved for a tourist visa. Your approval odds are lower if you are intending to meet a fiance. Unless the alien visitor is independently wealthy, or have other major strong ties to their country, approval is highly doubtful.
