I am your size, basically. 5'11", 180 pounds and Mamba 8.1 has been my main creeker and river runner. I love the 8.1.
At our weight, the Mamba 8.1 is perfect for learning, and for running everything up to Class IV creeking, and is great on bigger water of any rating.
Especially starting out, I would stick with it--plenty of volume to run everything you'll be running for a few years. If you progress to a point where you're running IV+ steep creeks, you'll probably want a bigger boat.
For me, the 8.1 has felt too small on two occasions, and that was 11Mile Canyon at very high flows (class IV creek) and on Lower Poudre Narrows (steep creek, V-) at higher than average flows. I've run other class IV creeks (Boulder Canyon, Lower Clear Creek, etc.) and its been fine. I don't currently run class V creeks and I'd like to get a Mamba 8.6 or a Nomad 8.5 or 8.6 (or similar sized creeker) for those occasions where I'm on a challenging creek or stepping up. But will be keeping the 8.1 for everything else.
To answer your second question, your feet should be solid and flat against the bulkhead without a lot of wiggle room. Your foot brace should push your knees comfortably up into the knee pads and you should have to push hard on your feet to get your knees out. Between the backband, hip pads, thigh braces, knee pads and bulkhead, you should be as snug as possible in your boat without things falling asleep or cutting off circulation. You should find that even when you're snugged in, you still come out of your boat just fine when you need to but it will be a bit tricky to get in to your boat initially. I would recommend putting at least 2 of the layers of foam that came with your foot bulkhead if not all 3, so you have some cushion for the pushin.
Also from my experience, you'll want your seat forward of the center point so start with that and set your foot position accordingly. I have ended up with my seat all the way forward so I can sort of lean back against my backbrace and still be fairly balanced. If you have better posture you may not need it all the way forward.