My newly adopted 2 y/o cat might be sick and I need advice please help

Hi, I apologize in advance - this is my first time posting on reddit and I’m still a little unfamiliar with the formatting. I also may over explain a bit but I’m just worried about little details being important, I will have a TLDR at the end <3

BUT I just adopted two cats yesterday and this is my first time ever owning cats. I have a 1 y/o male black cat named Renfield and a 2 y/o male tabby cat named Leno.

I’ve noticed that both cats seem to be adjusting to their new home wayyy easier/quicker than I would’ve expected. Renfield has been walking around like he’s lived here forever and Leno is very loving and snuggly. Although, I’ve noticed that Leno hasn’t even touched his food since he got here (at 7pm yesterday). I asked the animal shelter that I got both cats from what food they’ve been eating and the volunteer told me Purina One. I’ve never bought cat food before so I didn’t think to ask what kind, so when I got to the pet store I ended up buying the chicken flavor based on recommendation. Renfield has been eating it very well, Leno has not. I assumed I must have gotten the wrong kind so I called the shelter to get clarification. This time I spoke with the owner and he confirmed that the cats were eating Purina One but they’ve also been eating several different kinds/brands of kibble. They weren’t able to tell me what flavor or anything else but the owner recommended me to buy Redford (but he only mentioned the fact that it was cheap when he recommended it to me, he didn’t say this is one of the kinds they were eating). I couldn’t find that brand so I ended up buying a different flavor of the Purina One. He still doesn’t seem to be eating it although he ate a few treats and a Churu Salmon stick.

Then just a few hours ago I realized Leno was having diarrhea and seemed to be extremely less energetic than he had been before. He’s also been in my bed since this afternoon. I understand that I haven’t had him for long so I can’t make any assumptions about his personality but I am concerned that he is possibly sick. I’m also worried I’m being quite paranoid and he is just trying to adjusting to the home. I haven’t picked out a vet yet and I haven’t started my pet insurance policy so I’m worried that my paranoia is going to result in unneeded vet bills, but on the other hand I would rather be safe than sorry. I plan on contacting my family’s vet in a nearby town to ask for advice but I was hoping to get some advice sooner so I can try and relieve some of my anxieties lol thank you for reading!

TLDR: I just adopted 2 cats yesterday and it’s my first time owning cats. I noticed that 1 of the cats hasn’t been eating so I tried to change the flavor of the food but he was still not interested. Although, he did eat some treats and one of those salmon treat sticks. Then, today I noticed he was having diarrhea and then all of sudden I felt his behavior extremely shifted and he was much less energetic. I’m worried that he’s sick but I’m also worried I’m being overly protective/paranoid. I plan on contacting my family’s vet although I was hoping to get some advice a little sooner so I can try and relieve some of my anxieties