Brakkians can extend strands of themselves onto ether a lifeform or any entity which contains and/or emits energy.

To Brakkians, each type of energy has different "flavors" to it, so siphoning from an engine core is certainly not the same as siphoning from a lifeform. Organic energy is far more desirable... it "tastes" better and is easier to draw from.

In rare cases, the Brakkian can have a fully functioning organic guise which can act as an energy storage container so that movements require no effort from the Brakkian except for when the body becomes depleted.

The Brakkian can remain inside the guise until they die. There's no limit there so long as the guise has a mechanism to allow energy transfers from within (in other words, they gotta be able to "eat").

Secondly, whether the Brakkian decides to procure a mechanized, organic, or a mixed guise is up to the Brakkian and all types have have their advantages/disadvantages. Guises can assume any shape depending on the base materials used. In rare cases, multiple Brakkians can pilot a single guise to allow for things that aren't normally possible with just 1 Brakkian.

Brakkians have definitely raised a lot of debates in the way some of them choose to construct their guises (specifically, re-animating deceased lifeforms). Other debates include things like what is acceptable for a Brakkian to use as a guise? Mechanical guises can easily conceal highly dangerous weaponry or consist of appendages that could easily crush/slice/whatever other lifeforms... so how does one make sure that no Brakkian can enter a public place in a dangerous guise? Guises are costly and time consuming (remember, they don't live long) to construct and adapt to and are necessary for the Brakkian's safety (they are VERY fragile creatures), yet should that be an excuse to allow them to potentially pose a threat to others?

But yeah... it is quite an interesting social area to explore in the Delura series. I love stuff like that!