hana,
I'll start off by saying I am a HV and not a paediatrician. However, I am NOT a big fan of centile charts as I often feel they are a tool to beat parents with rather than reassure them.
I have only seen one child in the past 4 years with a true "growth disorder" and in that case the Mum noticed before anyone else that something was not right. Centile charts are only ever a guide and you have to look at the whole child and all the other aspects of development.
There is research which looks at growth and makes links to adult disorders, however, alot of that research at the moment involves children who are overweight.
Children who have true eating disorders can be underweight and the only way of adding calories is to give calorie rich food ie: often fatty and sugary food - it's a quick way of adding calories and helping a child to gain weight.
However, at 7 months she's still young and alot can change in 6 months, I'd go with your DD and offer her what you are having (if she wants it), follow on milks for the iron, plenty of fruit and veg etc.
It's still very early days and under current guidelines (weaning to start at 6 months) she's still a beginner - offer calorie rich (rather than specifically sugar and fat rich) foods - bits of Avocado for example are both calorie and nutrient rich and they also have a high number of "good" fats. Mashed banana is good too.
Cheese is okay if your DD likes it (also calorie, fat and nutrient rich) and a sauce over veggies is very palatable to many babies.
I suppose what I am saying is go with your daughter. If your older daughter is small as well then it's likely that they are just that build and that is okay. I have seen families where a baby has followed an older sibling almost to the letter in growth - the first child investigated (and found to be fine) and the second one just seen as a carbon copy of his/her sibling and not worried about (unless the parent's were worried).
Anyway - have banged on too long - I would agree that a paediatric dieticien might be a better bet - they are much more clued up about growth (and infinile better than HVs). It would be worth asking for your DDs growth to be plotted on a thrive line as these seem much more accurate.