The rules for #mini8 are identical to #8ball. The only difference comes in how you rack. Instead of seven stripes and seven solids, you only rack four of each in a diamond shape with the 8-ball in the middle. The numbers on the solids and stripes doesn't matter but the points of the rack should be opposite with one solid and one stripe on the side corners and one of each on the front and back corners. This is how it looks...
#mini8 is a good game to play when time is scarce. I like to use it on the losers side of tournaments when we get more players than the number of tables can handle. Having too many players can be a good problem to have, but not if it means that the tournament won't finish before the venue closes.
I run a scotch doubles tournament every Friday that averages ~22 players. One week it landed on the birthday of one of the bar's regulars and drew a field of 38! I knew we couldn't keep the normal double Elim #8ball format because it's already a "race to one" and the bar only has one table. Instead of going to single elim, we ended up playing #mini8 on the loser's side and finished just in time.
Although #mini8 may sound vastly easier than normal #8ball, in my experience it doesn't translate to every game being a runout. Of course there are more break and runs, but a lot of times a game ends up being a few innings and has some safeties, two-way shots, etc. It's like #8ball with the beginning stage cut out. For instance, during the the tournament mentioned above, there were 17 games of #mini8 without a single runout. Granted this was scotch doubles, but there are a lot of good shooters (and very few slouches) in this tournament.
Have you ever had all eight of your teammates show up for league practice and you all sit around barely getting in one game each? Well, next time instead of cramming full games of #8ball into the half hour allotted, why not save time and play some quick racks of #mini8 instead?