This applies to gallery shows, folios, books, whatever you have in mind. Any time you're putting a group of pictures together to make some sort of statement in any format whatsoever.
If I said anything at all smart in my book, it is this:
The approach you take to assembling a group of pictures is much the same as the approach you take to assembling a picture. Consider the balance, the variety, and the unity of the work. A portfolio can be balanced, in that it hits all the relevant themes about equally. It can have variety (or not). It can hang together as a unit, or not.
If you use a different theory of composition that your favor, translate its principles into things you can say about portfolios, and then apply them. It's the same thing at a higher level of abstraction. You might choose to contradict the individual pictures in the construction of the portfolio, or you might choose to repeat the same ideas, it's up to you. But choose, and choose for a reason.
I'm not even going to cite the portfolio that inspires this, suffice it to say that the artist has assembled a handful of more or less identical black and white photographs, and created possibly the least interesting portfolio I have ever seen, and is now wringing his hands about a picture that doesn't fit in at all. If the portfolio is supposed to be a dull collection of indistinguishable monochromes, then dump the color picture. Duh. If it's supposed to be more interesting, then spread yourself a bit and maybe the current loner will fit. Duh. If you still can't make it fit, then dump it. For the third time, Duh.
A portfolio should almost never be simply "the best frames", there's no room for ebb and flow, for rhythm, for highs and lows. That's a symphony in G major consisting entirely of G major chords played fortissimo in 4/4 time. Nobody wants to listen to that.