Advice on Puppy/Older Dog Adjustment

I have an almost 15-year-old female Aussie Cattle Dog Mix, and we just adopted an 8-week-old male corgi/GSD mix. We wanted to help our girl have a buddy to keep her young, and she has lived with other dogs in the past and settled into a "roommate" relationship with them, but it has probably been about 10 years since then. She has done well so far, only giving the puppy little grumbles or small nips if he gets in her face, but letting him sniff her and her food bowl, and use the shared water bowl. She has mostly just followed him at a safe distance as he explores the house. She free roams overnight and when we're gone, but until he is fully potty trained and a bit older, we have him in a play pen that zips closed.

I am wondering if anybody has any tips for helping them adjust to each other, be safe alone, and make sure that she doesn't feel replaced or forgotten? Also, tips on food boundaries would be amazing; he is food OBSESSED, which is proving to be a challenge with her typical habits of grazing on her meals over the course of a few hours. She's very independent and has arthritis, so the typical advice of extra snuggles, play, and walks on her own doesn't feel like it will apply; honestly her favorite thing to do is just sit outside in the sunshine and enjoy the sounds of nature, and we've been giving her lots of chances to do that as well as treats whenever he gets a reward on potty trips, and plenty of praise when I see her allowing him to come up close to her without issue.