I'd go with the 75. Same length as a standard 55 but a wider footprint, which gives you more flexibility with fish choices if you change your mind down the line. I know that my tastes have changed considerably over 40 years. The wider footprint also gives more surface area, like you mentioned, which is always good.
Then again, given a choice, I will always go with the bigger tank. You are the one that needs to be happy with it.
Inline heaters are generally lower wattage than in tank ones, I don't think it should be much of a problem. I like inline for a reason you don't need to consider, they're where large rowdy fish can't bang on them(I was always afraid my oscar was going to break his)
Overfiltration is good, truthfully, if the current doesn't bother the fish, I believe there is no such thing as overfiltration. I run 8 to 12 times turnover in all my tanks.
Hopefully this is more in line with what you were asking. Yes, you can definitely go with a smaller tank with the filter and heater you have. With the heater, I wouldn't go too much smaller, as it will cycle more frequently and probably shorten the heaters lifespan some. Probably not a lot, but some, and some will depend on whether the thermostat is electronic or bi-metal. Bi-metal is a moving part, very small movement, but movement, it would be affected the most, but you should still get a few years of service out of it. I retire mine at ten, for normal service. Doing this I've only had three fail in 40+ years, with usually at least three tanks running.