No. The 6 mth thing is not a 'guideline'. It is described as 'guidance' and ' recommendation' which is somewhat stronger and it is in use worldwide including the UK. It is likely to suit the nutritional needs of the majority of babies. I don't blame mothers for not knowing this, but I do blame health visitors for passing on rubbish information, at the expense of optimising babies' nutritional health.
It does not vary according to the size of the baby, either. Why would it, when solids do not add to the calorie intake? Solids earlier than the recommendation tend to replace milk, not add to it.
Babies' tummies rumble all the time It does not mean hunger.
I don't think there will be massive health differences between babies intro'd to solids at 17 weeks or 20 weeks, and the ones who have solids later to be honest - though for some susceptible babies, there could be. The point is that babies really do no need anything else but milk for 6 mths, and as it is a hassle, and an expense, to feed them more stuff, why bother!?
Alibobster, your HV sounds particularly inept, sorry - everything she has told you is wrong!!
OP - your baby is probably best off having no more food than she is having at the moment, and ditch the rusks