At low to mid level, the memory cost for class abilities will prohibit you from loading abilities outside one main tree. However, as you get higher in level and get more memory space, you can branch out in to other trees. At level 46, I currently have an Upgrade Master/Doctor loadout which goes most of the way down both trees but leaves out the less useful side-abilities which Sieges mentioned. To really customize your skills, pay close attention to where you put Attribute Points in the Upgrade Panel. Again, these will mean more at higher level. You can run a mission to get a Stat Hack to re-set your Attributes to level 1 and re-apply all the points, so you'll not be locked in to your first choices. Also, the clothing you choose will give a unique set of Influences. Other ways to customize MxO for your tastes include Macros and the layout of your Hotbars. There are a great many choices of Abilities, Attributes, Apparel, Macros and Hotbars to satisfy even the tweakiest of tweakers.
