I am not a big fan of rusks myslef! so probably not the best one to be answering your question. Rusks are a combination of foodstuffs, as opposed to the single foods we are supposed to be introducing in the early months - you know, the 'one food at a time with 5-10 days in between each'. And they have been known to cause choking too. Sooo, my idea would be to wait until much, much later than 6 months.
Babies do like to chew on anything and everything around this age, but I think it probably safer to give 'chew-toys' instead, and save the food for when you give it from a spoon - so you know exactly what food they are getting when! and you can keep a close eye in case of choking too... but that is just my opinion on that one!