I think everyone will tell you that rusks aren't recommended because of the sugar content, even the reduced sugar ones. But I'd personally say everything in moderation and you'll be fine.
There was an article in Mother and Baby this month, that basically talked about how everyone is beginning to take the feeding guidelines to extremes, there were mums featured who made their family live on just raw fruit and veg and people that just didn't realise that their babies were malnourished!!
Basically it said that some amount of sugary foods are fine, and there really are worse things we can do for our babies. Literally there are mums out there that are being advised to put double cream in all of their meals, a spoonful of jam into yoghurts etc etc, so I don't think a rusk a day is going to do Daisy any harm.
If she enjoys it, is getting her milk because of it and has a good, healthy varied diet, then you'll be fine.
You know best, and like you rightly point out, fromage frais are also full of sugar!!
On the iron topic though, it's worth knowing that iron from cereals, things like rusks and leafy green veg is much harder for your little one to absorb than iron from red meat, so as soon as Daisy is ready/old enough it would be worth introducing some kind of beef dish...also you should try giving her something that contains vitamin c when you give her foods containing iron because it helps the absorption.