A 75 for an angelfish is fine; I think your current stocking is fine with an angel though SAE can get quite large. 6 adult SAE in a 75 is going to be an issue - both in terms of fish per inch; but more so in swimming room and bio film. I don't think people realize just how fast SAE can swim and just how large they get. I intend to keep 4 in a 550 that is 10 feet long.
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The only require requirement for an angel is that the tank be at least 18 inches tall (21 or 24 are better) and at least 30 gallons - while larger is always better angels are not super active but if spooked they can move very fast. Females are normally significantly smaller than males and can fit in a smaller aquarium but it does come down to individual fishes as there are small males and large females. This is probably where disagreement in aquarium sizes form. I believe a 29 is large enough for 1 male for a short period of time (3 or 4 months); but way too small for a perm. home; for a female a 29 is plenty large. For breeding a pair it is 'ok' but i would remove them eventually. In my 120 i currently have 5 angels - 2 pairs and a floating something (I think a female but not sure). There is a fair amount of bickering but the stronger female has chosen one side and for this particular female her area is not too large and she is not too vicious (I've had far far worse than her); the other female took me by surprise when i realize she was a female and she is quite passive and lovely. When i move i'll split the two pairs up - one will go in a 550 and the other a 450 - the floater i havent' decided what to do - she or he has been getting pretty beat up recently so i'll figure something out - if i have to i'll move her into a 29 till i move (8 more weeks if i am lucky).
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Anyway i'd keep an eye on those SAE; it usually takes 9 to 18 months for them to become adult size (depending on age purchased). While they will keep growing once adults it is much slower and they are not small fishes.
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As for number of angels; sex is everything; 5 males will do fine; 5 females might do ok; more than 2 if you have a pair usually fails but it depends on individual fishes. Sexing young angels; dream on. Buy 5 - flip a coin and in 9 to 18 months expect to get rid of 3.
