Pretty much this. You can sort of peg her gun on those, too (via the hole in the center of the 5mm post) but run the risk of breaking them.
Okay, as promised, here's some shots with RID Arcee 'wearing' FE Arcee's blades: and vehicle mode: As you can see, the tips of the blades stick out further than the flat of Arcee's foot, but despite this, they actually add some stability to her. Having said that, I advise against putting the FE blades on RID Arcee unless you plan on leaving them there; as I pulled them off, I got some stress marks on the thinner of the two pegs on one leg, and both blades felt like they might break the pegs as I was taking them off.