Great, odd that you being a FF fan you haven't yet played FFVII :P I've played it first 11 years ago, and still play it today ;).
About the materia, you should focus on character building materia, putting "jobs" on your characters as you see fit. For example, you should focus Aeris or someone who has a lower HP and is more prone to be on the back row, with tons of spell casting materia. Someone like Cloud, Barret or Tifa, you should focus more on Command and support materia, to build up a better attacking pattern with them.
Also there are the mixed characters like Red XIII, Vincent, Cid and Yuffie who can play both roles in a good fashion, those you should mix with a decent amount of spell and command.
Also, make sure to spread your summon materia along your party, since those are the most HP and Strenght penalty materias, you don't want to stuck one character with all summons or he'll be OHKO for most boss battles.
About the All --> materias, you should use them with spell materias in double sloted weapons or armor. If you connect them in a double connector, you'll get the spell xAll enemies effect on that particular spell. There are tons of combinations to do, one's that are intrinsically complex but that makes for a monster of a character.
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
