9 kilos/20lbs is the minimum weight for facing forward, I wouldn't do it yet - her neck wouldn't be strong enough to support her head if you had a crash. The advice about sitting up unaided is outdated, we have new guidelines now :)
It's best to keep them rear facing for as long as possible, if you don't want to buy a rear facing second stage car seat, then that means keeping them in their baby seat until they outgrow it. Contrary to popular belief DCs can often fit in them past a year - DS was 18 months and he's fairly tall!
You'll know he's too big for the seat when the top of his head reaches the edge of the seat shell, or he reaches the weight limit of the seat, which should be printed on the underside, but for most UK-sold car seats is 13 kilos or 29lbs.
It doesn't matter if their legs hang off the end of the seat - they usually bend them when sitting in the seat anyway, and bear in mind it's not a good idea to put babies into car seats wearing coats, because it means that it's impossible to tighten the straps safely enough. So if you're finding it hard to squeeze him into the seat wearing a winter coat, drop the coat and put a blanket over him once he's strapped in instead.
HTH :)