First where do all waste from the body come from.
Imagine the suit as a heart who can take power from nuclear, coal, gas solar and wind. Well imagine your body as a solar cell. If you eat you get "sun" energy. If you dont you dont get any. You still have the solar panels there just in case the sun comes up.
In the same way the suit has probe up your ass (from what I understand is miniature). The Suit is built to interact with "normal" people. Normal people dont wear the suit without eating. They eat, sleep etc. The suit uses those excess. basically the suit followed its programing and is ready to use any waste from the body. There is none for now; but do not confuse this with the only source of power, its just an 'extra' source.
Assimilation: This is hard to explain without giving spoilers:
SPOILERS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The Suit is a copy of Ceph technology. Meaning the people who build it even Hardgreave didn't know how they work completely. Like copying a jig zaw puzzle. You can copy piece by piece and know they all fit together. If you take one out you then realize the final piece doesn't work and doesn't look the same as the original. So you blindly copy piece by piece, not knowing what individual pieces do... but knowing taking any of them out causes trouble.
Because of it, the suit even though "modified" to fit a human form it still has the underlaying programing of cephalopod. cephalopod's had a unique central nervous system, in which they dont have a "central" part. THeir tentacles have as a complex connection as their main body. Hence they are more intelligent as well as incredibly flexible. They can control all their suction caps one by one. Imaging having a finger every inch of your body and being able to control each one independently. Hell most have trouble using their ring and pinky independently

.
So to explain that. The Suit while having "human form" is still trying to rewrite the "host" to fit the underlaying code. which is that of cephalopod. By assimilation it means the suit finds that the central nervous system of the host is ... "incomplete" for lack of a better word and its trying to fix it based on its core coding. IN other words is turning the human central nervous system into a copy of the ceph, extending the neural network to every piece of your body in equal amounts.
That is what assimilation means... and that is one of the reasons why the term "dead man walking" came from.
Long story short: Suit to assimilate needs to have a compatible host based on its ceph central nervous system original coding. So its extending "your" central enrvous system to hundreds of times its length.