You can use a slip float if you want, in fact people were doing that long before the float and fly was invented. You just can't really call it float and fly if you're doing that, that's all. Float and fly is a specialized technique that relies on the special bobber and the 3-way swivel. That bobber will lay over on its side if there's no weight underneath it. You get a fair number of bites from fish that are coming up under the bait, so they're not going to make a float go under the water. A float and fly bobber will lay over when you get those bites, whereas a slip float is just going to sit there. So go ahead and use your slip float, it's a lot easier and cheaper, but it's not accurate to call that float and fly fishing.