Developmental differences between grey and fox squirrels?

[furryscaly]

Hi, all. This is my first post here. I do small-scale wildlife rehab and animal rescue in my local area. Although I've done orphaned raccoons, rabbits, birds, etc, and have done injured adult squirrels, this is my first time rearing infant squirrels. Thankfully my experiences working with other critters carried over, and it hasn't been terribly different from any other baby in need of frequent feedings and care. Of course there will be specific differences in rearing squirrels vs other animals that I've had to look up, such as how old they are, how much to feed them and what brand, etc. When I got the call to pick them up they were about 3 weeks old, and I'm happy to say they are now about 9 weeks old and doing well. Ok, so after that long-winded intro, here's my current issue:

Where I'm at we have fox squirrels (Sciurus niger). They're a larger cousin to the grey squirrel, but are overall similar. They're heftier, with shorter ears and orange highlights instead of white. The problem is that pretty much all information on rearing orphaned squirrels on the internet pertains to greys, not foxes. I assumed any differences in development and care would be minor, and I made only slight alterations, such as feeding them just slightly more than what is recommended for greys. However, I used descriptions of grey squirrel babies to determine the rough age of my fox squirrels when I obtained them, but based on that starting point (3 weeks old), they would be 9 weeks old now, but do not seem as developed as greys of that age. For example, when I got them their eyes were closed, with only thin, peach-fuzz hair covering their bodies, and hardly any hair on their bellies. But now, 6 weeks later, they're finally starting to explore a little, but still sleep a lot. They occasionally hop a little, but don't run or jump much, and I haven't observed them sitting up on their haunches at all. They nibble and eat hard foods a little bit, but still are very eager to take formula. Their tails are bushier, but nowhere even close to an adult's, and they've only recently started carrying them over their backs somewhat regularly.

Have I misjudged their age? And if so, does anyone have any pointers as to how old they may be? Also, what about care requirements for whatever age they are at? Because they're either 9 weeks old, and then apparently 9 week old foxes are much further behind 9 week old greys, or they're not 9 weeks old at all and I can't accurately tell how old they are. At what point do you think they will be getting closer to weaning? From what I'm gathering it seems fox squirrels may have a slightly longer developmental period than greys, but I could be wrong. Adult fox squirrels weigh up to 2.2 lbs, depending on locality, but mine are currently 6 oz on an empty stomach. I'm not too worried that anything negative will happen, but it would still be nice to know more. I'll see if I can add some photos of them when I first got them on March 2nd vs what they look like today, April 6th.