Welcome to the HOD Board, and thanks for sharing about each of your dc!

HOD has age ranges to recognize the fact that age is only one determining factor in placement. All 5 yo's are not the same, just as all 6 yo's, and all 7 yo's are not. So, looking at the placement chart with just your dd in mind, where does she seem to place best? Here's a link to the placement chart...
http://www.heartofdakota.com/placing-your-child.php
This part of your post...
Her attention span and ability to ask and answer questions about stories is pretty good for her age (we read a lot!). She already knows all the preschool stuff - letters, sounds, numbers, shapes, colors, etc... Her area of struggle (in her attempt to keep pace with ds ) is writing. She can trace letters/numbers ok, but cannot write many of them independently yet. She loves tracing, coloring, and mazes, so I anticipate that she will improve some by this fall. Will she do ok with LHFHG? Up till now, we have usually only done "formal" preschool 3-4 days a week. I am planning to do 4 regular days next year since ds will be in his official K year, but don't mind slowing things down a bit if needed.
... makes me think that as long as you take it slow, dd could begin LHFHG along with your ds, especially considering he is only 5.5 yo and has CP/autism to consider in placement. I also love your idea of doing school 4 days a week. Do you know you could do LHFHG, Beyond LHFHG, and Bigger Hearts all 4 days a week, and this would take about 4 years or a little less. This would progressively move your dc through the guides at a pace they can both grow into, and give you 4 days of teaching each week rather than 5.
We did LHFHG with both of our sons when they were 5 yo.

They loved it, thrived with it, and even began reading the Emerging Reader's Set from Beyond halfway through LHFHG. One of the biggest blessings of HOD is that it is not a cookie-cutter, one box fits all kind of curriculum. So, when considering your dd, where does she seem to fit in the placement chart, and looking ahead, can you just do school 4 days a week to allow the dc to have more time to mature as they move through the guides? I'd say proper placement depends on all of that. HTH, and I'm looking forward to what you have to share about this!

Dc 18 months apart can certainly be combined with HOD - let's just figure out if that's what's best and if that's what will work down the road too!

HTH!
In Christ,
Julie