Is this chick, or are there two you're concerned about, behaving normally? Runny around at top speed, crashing suddenly for a brief nap, and then off to the races again? Or are they lethargic?
I am unable to see anything unusual in either of your photos. Down is not feathers and it appears to disappear when wet. If the vet is red, irritated, and swollen, I am not able to see it, but will have to take your word for it.
Chick vents swell when rubbed since the tissue is very thin. Put coconut oil on any swelling or irritation. It will also help keep poop from sticking.
Many new chick people mistake brown puddly cecal poop for diarrhea when it's a normal, if odious, feature of chicken poop. Diarrhea is very watery and doesn't puddle, it spreads thinly everywhere. Also, a chick with diarrhea will be lethargic due to dehydration. It happens, but it's rare.
Don't worry about grit if all you're feeding is chick crumbles. Chick grit is like sand and can be bought at any feed store. You can offer grit at any time, but no need to mix it into food.
If you want a more nutritious form of chick feed, try fermenting it. Most chicks adore it, and it will usually firm up poops. To ferment, add a cup of warm water to a cup of dry feed. Add a teaspoon of ACV, stir and let sit in a warm place over 70F, stirring a couple times the first day. By the end of the second day you will have a fluffy, yeasty ferment, smelling pleasantly like yeast. As you run low, add more water and feed to the desired consistency come evening. By morning it will be ready for the chicks to eat again.
