hey rothko
given the information you have there about them playing fit/fold and never adjusting, then fine it's probably ok. however there are some flops that absolutely smash a preflop calling range and therefore they will be extremely borderline cases even against a fit/fold opponent who doesnt adjust. also, people will and do adjust - often people overadjust by default and float everyone WAY too much and so it will be hard to really follow this strategy against everyone without consideration. instead it would probably better to broaden your understanding of cbetting to the point where you can identify situations where it is ok to cbet 100% of your range and situations where it would be terrible to do so.
so to figure out your required fold equity, just use this formula: invesment / (pot + investment) * 100%
investment will be your cbet and pot is the pot before you cbet (or 3bet etc). this will give you the frequency that you need villain to fold to at least break even with a bet. for example: 3bb / (5bb + 3bb) * 100% = 37.5% so we need our opponent to fold at least 37.5% in order for our 3bb cbet to at least break even.
next, give your opponent a range and put in a flop into equilab. click on the pie function next to the range and click the check boxes for all the hands that you think he will continue with vs a cbet. if he continues with this amount of hands, then he folds the rest. if he is folding a % which is equal to or greater than the required fold equity then a cbet of that sizing will be profitable. if you do this exercise a couple of times you'll get a feeling for not only how often people continue on different board textures, but you'll also get an idea for some good sizings.
about the 3bets, yes it will be in theory but if some guy comes along and 3bets you every time you open raise i'm pretty sure you'll notice and do something
but if he really won't adjust then just 3bet him to death i guess!