yikes! lots of questions there. i'll see if i can pick them out.
-550 motors have a longer can than a 540. 540 motors are typically 50-53mm long. 550's are anywhere from maybe 60-72.
-heat is a function of amp draw. if you gear the motor too high (which will cause a high amp draw), any motor will heat up.
-mmp has sensorless and sensored operation (just like the RX8) and was designed as a 1/10 esc (although it can be used in some 1/8 applications). the MMM runs only sensorless and was made for 1/8 applications.
-1410 is a 3800kv 540 can. the 1415 is a 2400kv 550 can. if you go with a castle motor and will only run 2s lipos, then the 1410 is for you. if you want to run 2s and 3s, the 1415 is a better choice.
if you're set on a sensored 550, i'd rather have a viper VST XL. IMO they are the best sensored 550's out there. if you are looking for a sensored motor, the tekin pro-4 is a great little motor but very expensive. if you don't need a sensored motor, the tenshock SC401 is a 4 pole 550 can that is a great, inexpensive option.