Many say you cannot --- zebras, Yet, others will disagree and claim, they are sexed like the fuzzy dwarfs. So lets look at both, so you can decide.
If you Can --- a zebra, than the same rule would apply as the fuzzy dwarfs, One Male per tank, and the rest should be females. So, in your 90g, you could have One Male and Two Females, that would make a nice group.
To --- a Fuzzy Dwarf, adult male Dwarfs have larger heads, longer pectoral fins, and 6 to 10 bands on their pectoral fins, females only have 4 to 6 stripes.
If you, do Not think you can --- a Zebra, than the same rule would apply, Only One Zebra per tank. If you add two Males to the same tank, they will fight, harass each other, cause stress, even cause one or both to jump.
Kaye