Be careful you don't damage your batteries by discharging them too far. From what I've read Makita and Dewalt have the low voltage protection in the tools and Ryobi have it in the batteries. By using your combination of Ryobi tool and Makita battery you are potentially missing the discharge protection.
i'm using a Ryobi with a Dewalt battery, with an adapter, works fine. only £50 quid for the timmer and £20 for the adapter, ( £11 if you search ebay )
Was just looking at these. Do you happen to know how it compares to the DeWalt one, as I already have the batteries? Don't really have the need for the blades, just the string.