Yes, you can attach them to the buttercream. The royal will indeed get soft and "meld" onto the buttercream.
This is great if you're doing letters for the top of a cake, or sides, where you want it to look freehanded.
Not so great though, if you're wanting your designs to stay very hard. Most things WILL hold their shape, however--even if they do get softer.
--Knox--
Yes you can attach them and I've never ever had royal get soft on buttercream.
I made a cake for Christmas covered in buttercream and used royal to make snowflakes. Only 1 layer was consumed at the party and the other one was slowly eaten by DH and I the following week (with still some left at the end of the week). The royal snowflakes were still intact and strong, absolutely no signs of melting into the buttercream.
I've wondered since I did this if this is some sort of cake myth. 
Mine Royal accents never got soft either. I was told they would. They were on my cake for a good week- week and a half.
I actually saved my accents from my cake for my own use if I ever need some in a quickie!!! I thought why waste all that time I used to make them. Just don't store them in a fridge.
mine dont get soft either. The only thing I experienced once was a color flow piece. It appeared as if some grease was soaking through it and it looked a bit speckled. But it wasnt soft, so I dont know if it was indeed grease or not. But overall, it will be just fine!
I did royal icing letters once for a monogram. The icing behind the royal was buttercream, and after a couple of hours on the cake, the monogram letters were soft.
It may be because at that time, I only had a hand mixer (not a kitchenaid) to mix it with, and I didn't ever get the royal firm enough to begin with.
But along with that--I've read here on CC PLENTY about it getting soft as well--so I'm not the only one, surely.
--Knox--
My royal icing flowers get soft on buttercream, too....so Knox you are not the only one.
I knew I wasn't 10-87 just yet, Monica!!
I hear what you're saying--but I keep lookin' at that big ol' fluffy MAX. I just want to rub his ol' belly for 'eem! (that's TN talk for "belly rub." for the rest of 'yins...)
--Knox--
Whats 10-87? 
My Royal flowers on buttercream get soft after about one day on the cake.
Really Soft 2 days after. THey will fall apart when picked up.
Interesting the differences experienced. Maybe it has something to do with the length of time the mix is beaten or maybe how long they dried before being used.
I use a mostly crisco bc and my snowflakes were really thin. If anything, they jsut got frail from being in the fridge but not a lick softer than when I first put them on.
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