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putting $300 across 70hp is very cost-effective for eurorack ;-) when i need that last nudge to buy a harmonic oscillator or bark filter, i think of how expensive it would be to fill that gap with a lot of small modules.
2hp gaps are a no-go. you end up with a gap, blind plate or a small module you didn't really want, just to fill it. so first, i would rearrange things to get all the open space left in one place. it gives you more options in filling it.
personally, i try to keep the number of different brands limited, i don't like small modules either, and i like to have the colour-scheme a bit balanced (scattered that is, because it won't work to get one row black, the other aluminum, another grey) - then i try to get as much vertical lines between modules too. it makes it look neat. ofcourse i realise these are trivial matters, first and foremost it's about sound and not vision. but i can't help myself. as to how to fill the gaps, with what modules, that's up to you. like the previous poster, i would take certain one's out. but hey: it's not my case! you decide what goes in or out. have fun.