I'll be starting the 'all-pro' routine soon, and was considering hiring a PT for a couple of months to help me dial my form in.

Is it poor etiquette to have already chosen my workout plan, and only use the PT for teaching form? I'd obviously be upfront about it before they took the job.

I don't think so at all. I would be clear on your goals and I would also make sure that the trainer is confident in his or her ability to teach you the correct techniques.

I worked with a trainer for 12 weeks and at the end of those 12 weeks, after repeatedly telling her I wanted to lift heavy, I told her I couldn't sign up for a another package due to financial reasons and was going to try a program I found online. Naturally she wasn't very happy with me but was professional enough to not be pissy about it. I did ask her if she'd be willing to work with me two more times, I paid her of course, to check my form on the Strong Lifts routine. She did and she was really good about it. She was very conscientious about making sure I was doing the lifts safely. Any trainer worth their salt, should be professional enough to take you on for a couple of sessions. They may try and discredit the program or try and convince you that they have a great package that would be better for you, but if you tell them up front that you just want two sessions for the purposes of making sure the lifts you want to do are being executed properly, they should be fine with it. You're saving them from having to develop a routine for you so it's actually pretty good money for just watching and correcting your form.

I provide personal training.

For some reason, I take that literally.