cos at slow sync, your exposure will be depended on the ambient light and not the flash, so that it can be effectively a fill-flash. The only drawback is that you can't control the flash intensity with slow1 or slow2, so you can't control the extend of the fill-flash. Maybe can try experimenting here and there first.
Using slow1 or slow2 should have not much of a difference since for fill-flash, the shutter speed is fast enough to resolve the different between front curtain and rear curtain sync.
As long as when you are in M mode, you take the shot according to the metered exposure (or +- a bit up to you), and not adjust the exposure for the flash, you will be able to use slow1 or slow2 as fill-flash. Of course, if the ambient light is low, such that the flash will overpower it, then it won't function as fill-flash anymore.