All of our carriers fit differently, and depending on the wearer's body type and baby's size, one will likely stand out as being more comfortable. That being said, if you are having under arm rub (depending on your bust size), you are likely pulling the chest clip too tight, as well as placing it too high, and the shoulder straps are too loose. Try loosening the chest clip & lowering it a little, and also tightening the shoulder straps.
Be sure that he is old enough (4-6 months) and comfortable to spread his legs around you comfortably. He may be in that transition period from the infant insert... if baby has great head control, but still can't spread his legs around you, you may take the pillow out and seat the baby just on the pillow (without the "taco" part). Also, make sure that baby has a good deep seat in the carrier and that the weight is not resting on his legs, but rather his bum.
Many women have safely used the Ergobaby carrier throughout their pregnancy. However, this is very much an individual preference and we recommend you consult your health care practitioner if you choose to continue to carry your child beyond your fifth month of pregnancy.
Any babywearing can be warm. That said, many parents like our Cool Air Mesh carriers, as it has some mesh wicking to help with breathability.
Also, here's a great post about Babywearing in Hot Climates: http://ergoba.by/WbBN84
And our friend at Carry Me Away also wrote a great post with some tips, here's her "Guide to Wearing your Baby in Hot Weather": http://ergoba.by/1405UzK