I did have the one betta in with my male guppies the Orange Flame (which are like a highlighter orange coloring.) and it did just fine. So it cant be said they only get along with females or pale/plain colored guppys. The reason I moved it out was because all the males kept trying to mate with him. So before listening to all the rumors and myths about keeping guppys and bettas together I suggest you get a betta barrack or something to keep it at a safe distance so you can see if it has any aggression.
Another way to make sure you get a calm betta is to buy from a breeder that already knows the temperment of the fish.
I do agree that you have to be sure your not over stocking, also depending on the strain of guppy you have they will vary in size. Such as, my moscows are huge but the topaz are about an inch and a half tops. So I can safely have more topaz in a 10 gal then I can Moscows if you go by the size and gallon chart.
I just dont like when people who have not kept guppies with bettas automatically say it wont work, only because I know it does from experience. I've had many bettas with my guppies over the years but I currently only have two. And yes I know now there are some that are too agressive but thats why you need to be very selective when getting one. (Smallest tank Ive had the together in was a 5 gal. with 3 topaz and one female betta.)