I'm not so sure if it's suitable for me. Mainly because I suffer from bad anterior pelvic tilt, hip shift, weak quads, and rounded shoulders.
None of these things are actually problems - either for lifting or otherwise.
Anterior pelvic tilt is the normal human pelvis orientation, with 80% of healthy people exhibiting it.1 It does not correlate with low back pain.2 And despite claims you might hear, it's also not associated with hip flexor tightness or abdominal weakness.3 It's also largely determined by the shape of your pelvis4, which is not something you can change. Here's an easily understandable video on the topic.
Hip shift is a concept that people have only started obsessing about in the last few years, but it's safe to say that there is absolutely no research or even mechanistic justification to suggest that it's harmful. At worst, it could possibly result in a slight loss of efficiency. It could be caused by one leg being a little stronger than the other, in which case focussing on keeping straight in the squat (possibly with vocal cueing coming from a coach or workout partner watching you) will likely fix it. Or it could just be a requirement of your skeletal structure, given that 90% of people have a leg-length discrepancy5, in which case all you can do is ignore it.
Weak quads can only be said to be the result of a lack of strength training, and for obvious reasons, are an indication that you should be doing strength training, rather than avoiding it.
Rounded shoulders are not associated with pain6 or risk of impingement7, but can be modified through exercise8 if you're concerned about it aesthetically. The rows and (optional) chin-ups used in Stronglifts would very likely achieve this.
A big guy at the gym told me I'm too weak to do the program, and should instead do more isolation to strengthen my body before getting on the Stronglifts program.
He has no idea what he's talking about. Stronglifts is suitable, without modification, for anyone strong enough to be able to lift a barbell. And even those not strong enough to start the program with an empty barbell could still do the program just by starting with lighter dumbbells instead. Focussing on isolation exercises first would certainly leave gaps in your musculature and completely neglect training the balance and coordination of compound lifts, arguably leaving you in a worse position to learn the compound lifts.
I can go +20-30 kg on squats/deadlifts but my form breaks down. Eg: My knees start to cave in when doing squats
This is normal. The hip adductors, which bring the knees together, also majorly help as hip extensors in the bottom of a squat, so it's very common for the knees to "cave in" when lifting near-maximal loads. For instance, see this world record 193kg squat at a bodyweight of 63kg.
stiff-leg when doing deadlift.
This is a little more of a problem, and usually occurs due to the back rounding before the bar comes off the ground. Most people who continue the lift in this posture struggle to lock the shoulders back at the top of the lift, because even though they can straighten their legs, they don't have the back strength to finish the lift by raising their chest and pulling their shoulders back. They often then begin hitching, where they bend their knees again and push them forwards under the bar, in an attempt to jerk the weight up. A simple way to treat this is to consider the lift to be failed if you can't fully lock it out at the top without hitching, and proceed as you would if you had failed at any other point in the lift. Then you don't have any incentive to just try to continue the lift once you have moved into this problem posture, and instead you are forced to work with a lighter weight that you are able to lift properly. (Where "properly" is defined as within the rules of powerlifting.)
I guess my question is: Is it ok to start Stronglifts even if you have imbalances in your body?