you don't have to start low, but it's really really REALLY important to get the form down for the lifts, therefore starting with lower weights is usually a good idea. If you can squat 135 5x5 with fantastic form, then there's no problem. BUT, if you have shit form (and trust me, there are a LOT of things you can do, and probably are doing, wrong with squats and deadlifts) then the weight you're lifting is useless as you're probably hurting yourself more than you're helping yourself. A lot of PTs don't know what they're talking about and will give you bad advice. Make sure you're going AT LEAST parallel on squats, and preferably lower. Work on your flexibility if you have to.
For my brother, who started 5x5 3 weeks ago, I had him doing a few isolation exercises because a little aesthetics never hurt anyone. This is basically what I told him to do (might be a little off because I outlined the program 4 weeks ago):
Workout A:
5x5 Squat
5x5 Bench
3xF Chinups/Pullups (He can only do one per set but it's getting better)
3x8 Biceps
3x8 Triceps
3x12 Reverse Crunch
Workout B:
5x5 Squat
5x5 Overhead Press
5x5 Inverted Row
3xF Pushups
3xF Planks
You might notice that he isn't doing deadlifts. Personally I'm awful at deadlifts, and I don't do them because I have actually tweaked my back pretty badly before and don't feel like risking it. Since I'm bad at them, I can't teach him proper form so I don't want him hurting himself either. HOWEVER, If you can learn deadlifts with proper form, they are an excellent lift to put on a bunch of muscle.
Point is, yes, you can add isolation lifts to 5x5 as long as you focus on the big lifts because those are the lifts that will be adding most of the muscle on your body.