A quick question: can I use Lactose instead of candi rocks towards the end of the boil for a Belgium Blonde? I have lactose left over which I want to use up.
It'll be very different. Lactose isn't fermentable, which will end up with your beer being sweeter.
Candi rocks are fully fermentable.
You can just use table sugar (sucrose) instead of the rocks in a Belgian blonde unless you need that slight caramelised flavour you'll get from the rocks (depending on their colour)
I've not used the rocks myself (I've just caramelised/inverted table sugar).
Darker crystals will impart a richer/nuttier/more bitter taste than the light ones. Largely, have a taste of them - that's the taste you'll get in the final product (minus the sweetness).
As far as I know lactose is a non fermentable sugar added to sweeten beers like milk stouts. Because lactose sugar is not converting into alcohol all the sweetness remains in the brew. Sugar drops or any other kind of sugar will be perfect to carbonate your beer.