Your poor dd - and poor you! Sounds horrendous.
Our DS is 6.8kg so still following his line below the 0.4th centile. He's also short for his age so not skinny though and has no other health problems so far. We never had to give him high cal formula (bf for just over a year, a killer as he woke at 1am, 3am and 5am every night until a few weeks ago 
). He is such a picky eater it drives me mad. I had that awareness of my mood being dictated by how much your baby consumes with DS1 and DD but with DS2, because of his size it is 10x as bad! We now rely on getting a poached egg mashed with butter into him every day (the only way he will eat egg) - he refuses bread, cereal, all fruit, avocadoes...but likes chips, baked beans, mash, pasta and pizza and will tolerate broccoli and peas. He is slowly getting better with food and drinks lots of milk.
Endocrinologist said pretty much what we expected - there are no obvious markers for RS but she will see him again in 6m to see how he is doing and will refer us to a geneticist to see if they want to test for anything. If he is still not catching up in his 2nd year then growth hormones would be a possibility. I don't think they would give gh to any baby younger than this, so you are right, there is no reason to rush your appointment. Also, putting a label on a condition really won't make much difference, DS probably doesn't have RS but may well still need GH.
Keep me posted as to how you get on. Good luck, it is so stressful I know...there are much much worse health problems for children to have though, which is what I try and remind myself.