With glide_size, the code says it rounds down (floor) when it's not an integer and needs to be seen as one. Older clients cannot support the floats natively, because 1) they didn't have the backend code to handle it, and 2) the message format had to be changed to allow for it--hence why that change was put into 509 instead of 508. But because the float can be rounded down, using it won't preclude 508 clients from joining 509 servers; they'll just have a subtly different glide_size.