There are many placement options within HOD, and not one hard fast set placement that is right when family dynamics are considered. I do think it is important to consider family dynamics - especially in larger families. Only you can know which combinations are going to work well, and the pp had a good option to consider.
Another option would be to have the 5 and 6 turning 7 yo do LHFHG. This would be a good option as it will give your almost 7 yo a solid balanced year in history, science, etc. befitting his age without requiring a lot of writing. I think this is an important consideration given he has difficulty writing due to his muscular dystrophy, especially looking down the line at the guides since the writing increases each year. You could have him do the Emerging Reader's Set from Beyond if you would like, as it is a wonderful reading set full of living books that progress nicely, and it has scheduled questions in the Appendix of Beyond. However, you can certainly use Abeka readers if you have those on hand and are enjoying them with him already.
Your 9 yo sounds well placed in PHFHG. Having had a poor speller myself, I will put in a plug for trying the HOD style of LA. Dictation, copywork, and R & S English grammar done CM style within HOD have steadily improved my ds's writing through the years until he now rarely makes spelling /grammar errors. It is easy to do and effective over time - so I'll just throw that out there.
Your 11 yo sounds well placed in RTR, but could do PHFHG with extensions if you would rather. RTR is a terrific guide I am certain she would love as long as she is truly ready for it. It is a meaty guide, so since she is new to HOD, you'll want to print out the first week of plans and really look over the "I" boxes to be sure dd is ready to take those on independently. Check out the "S" semi-independent boxes as well, to see if you can give her some direction and then she can finish that work out on her own successfully. Is she a strong reader? Strong writer? Able to work quite well through multi-step directions using the guide as a student planner? If you are answering yes to all of these, and you look over the week of RTR and she looks ready for it, I'd have her do RTR. I think this honestly would be easier. If you are thinking she may not really be ready, I'd have her do extensions of PHFHG with younger brother instead.
http://www.heartofdakota.com/pdf/rtr-we ... glance.pdf
If you do choose RTR for dd, we have found a midpoint meeting time to be very effective as an enjoyable way to work together. About 2/3 of the way through Wyatt's work, we meet together (me with coffee, him with tea

). I look over his work he's checked off in his guide, and I do the follow-up checks for each box (i.e. I ask him the questions, read the key ideas, have him give me his oral narration, have him read aloud his written narration and then we use checklist to edit together, look at his notebooking he's done, have him say his Bible verses for me, etc.). These follow-ups I mentioned are not all every day; they are rotated. I enjoy doing them and feel like this is such a "grown-up" way to approach homeschooling with this age of dc. He appreciates it and enjoys it too. I also do the grammar, math, writing, and any other "T" noted lessons in the plans at this time. It took us around 1 hour or so in RTR, and worked so well!!!!
Anyway, those are just some thoughts. What are you thinking? Keep chatting it through here, and we'll be glad to help until you have come to a peace with placement. HTH!
In Christ,
Julie