You sound very strong generally in upper body to me. Just under 5 minutes for a classic plank is awesome.
Leg press feels unhealthy for my spine. I have very strong legs but don't want to put my lower spine under undue pressure. Doing a an L-plank while leaning against a wall feels a lot more natural and doesn't give me any pain in my spine.
My classic plank is only 94 seconds. Again I don't want to push it much further as it feels very uncomfortable for my lower spine. I am hypermobile - I can do a crab, so I have the opposite problem to you.
Re: shoulder muscles - I expect there are exercises you could introduce to make improvements in your shoulder strength specifically. I use downward dog yoga position and shoulder presses mostly. I expect that heavier lifting with deadlifts is the way to go for improving shoulder strength.
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.