maybe the size of your pouch sling is a little too big for you? if the pouch is too big, the baby will lean against the sling instead on the wearer's body. the weight of the baby will pull down on the wearer's shoulder and this can cause shoulder pain. a good pouch sling is a snug fitting one which pulls the baby closer to the wearer's body. the pouch must be spread across the back and centered on the shoulder so that the weight is spread evenly on the wearer's body. i find this site
very useful. my new pouch is also a little too big than my previous pouch so it's hurting my shoulder too.
as for front carry carrier, i read that it's not good for the baby because the 1) the feet are dangling, which is not good for the maturation development of the legs; 2) the baby is made to sit on his crotch, rather than his bum, and this is not our natural way of sitting, plus all the weight will go down the baby's crotch instead of his bum and this is not good especially for baby boy; 3) the baby is facing away from the wearer, a position that can cause an over stimuli to the baby and the baby is unable to snuggle against the wearer. you can refer to this site
as for the kangaroo carry style, the bending of baby's legs is very natural, as the baby's legs are also bent when he was still in the womb for 9 months. as long as the baby is not crying, it means he is happy and comfortable.