vc -
I feel for you. I have had problems similar in the past. I'm sure your older cat feels very territorial and probably a little bit threatened by your new addition. Often, peeing and pooping outside the litter box is a behavioral thing, like a toddler throwing a temper tantrum.
LisaKay suggested getting another litter box. Have you tried that? When I got a second cat, I added a litter box and my boys actually used one litter box to pee in and one to poop in. Worked perfectly - -until I brought #3 home!

Anyhow, my oldest cat would sometimes poop right outside the litter box, I think because he was trying to indicate to me how offended he was about something. And he was very particular about his kitty litter - - wanted it clean (don't blame him) and it couldn't be the super cheap stuff (he's a snob)
I would suggest, other than adding a litter box, asking your vet about it. He/she may be able to give you suggestions to help ease the transition. Also, be sure and give your elder pet plenty of attention - - I'm sure she feels her home life has been disrupted by the new addition.
Having lived in a 1 bedroom, 1 bath condo myself, I found that those Air Wick air fresheners (the kind that plug into outlets) work pretty well for keeping odors at bay. Also (of course) scooping the litter boxes as often as you can; I tried twice a day, morning and night. When I was home, I would scoop as soon as they used it. Don't know if you know or not, but cats generally don't like those hooded litter boxes. They are great for us (we don't have to see the mess) but the hoods cause odors to get "locked in" and a cat's sense of smell is much greater than ours, so they may be picky about relieving themselves somewhere that odors are locked in.
Anyhow, I hope you find a quick and painless solution! Keep us posted!