I think that the scouts' problem is kinda relative. I mean as far as I know they we're selected from the actual legionnaires (during Claudius timeframe) at least 'till Marcus Aurelius and the Marcomanic Wars. I say that for this timeframe, the Romans have more then enough units. And on the other hand, a scout unit wouldn't be that helpful if you ask me, plus that you can alway roleplay and you can use a different unit for this exact purpose.