I want to adopt a dog? Please help?

Alright, so I want a dog REALLY bad. One that I can hangout with all the time and just have a constant companion. I had already set up to get a miniature australian shepherd... but for some reason my heart is guilting me out of it. I've gone to shelter after shelter looking for "my dog" but unfortunately it have to meet a certain criteria (which I really hate admitting). First off, I live in an apartment. So it can't get huge. Then I could adopt an adult dog (which I WANT to do) but I have a cat and I can't put his safety in jeopardy. There's also house training. How will I know if he'll/she'll go outside? Please help. I really want to rescue a dog.