If you rollerblade most days of the week, you'll probably meet the physical exercise requirements of most dogs. Your have a dog already, so you are already aware that it'll be your responsibility to take the dog out regularly for a potty. So, I think your set as far as that goes.
I have doggy-sit a black lab and a rottweiller (both adults; different owners). The lab stayed in my home, was calm and extremely well behaved. I stayed with the rotty at her house. She was and indoor/outdoor dog that would sometimes get rambunctious. She'd attempt to jump on me when I walked out to be with her in the backyard. Inside, if I was sitting on the sofa, she would want to snuggle close, often trying to sit on my lap. She was often ignored, so I think she was just starved affection. I believe if her owner had done a better job of training, she would have been a perfectly well-behaved indoor pooch.
In short, I don't think size nor activity needs would prevent you from having a good apartment dog, so long as you made sure to take care of the dogs needs.
In your situation, my concerns would be as follows:
I would worry that an aging dog, especially an aging Jack Russell terrier, may not like sharing his space and his people with another dog. I'd also be concerned that a big dog (or big puppy) could accidentally hurt the old guy even if the Jack was accepting of the new dog.
While people do it, I personally cannot imagine raising up a puppy in an apartment, especially if you are not a 24/7 stay-at-home doggy mom. My last dog cried loudly every night for weeks when we first adopted him as a pup. I bet that would not go over well with neighbors who are only a few yards and a thin wall away. And then there is the chewing (of your stuff and your landlord's property) that first year or so until the pup is done with teething and has learned what is and is not okay to gnaw on.
BTW, my last two dogs were both German shepherd mixes, raised separately but with similar outcomes. They both had 24/7 access to the backyard, yet they both preferred to be in our tiny house with their people. Once grown, they spent most of their time indoors being calm, loving, respectful household members. The first year or so with both dogs was an indoor challenge. They were big, clumsy pups. They'd slam into walls as they tried to run around corners, skid on the linoleum and bang into chairs or table legs, knock things off coffee table with their tails and all that other fun, hyper puppy stuff.