3. No. Shortnose Gars, while being the smallest of the true gars, is also the most skittish. It will at some point severely injure or kill itself by slamming into one of the walls at high speed when you do something sudden, like breathe.
1. it's probably the shortnosed gar, but i'm not going to go any further on this as i have no experience with them. the next smallest would be the florida/spotted gar. which gets up to 20"-24" in an aquarium, and then tropical gars which i think hit 24"-30" in captivity.
2. i dont think any of them stays under 18".
3. it would depend. a 75g would be fine for several spotted/florida , shortnosed or even tropical gars for perhaps 2 or so years. however they will eventually need a larger(6' but preferbly 7' long and 3' or so wide) tank. however it wont hold any of the other gars for very long.
a better alternative would be to get a 4' by 2' wide. i missed the part on it being 18"wide the first time i read the post. a 24" would then be able to hold it for 2 yrs or so.