my trail bike (only i've ever attached a rear rack) have a straight line from the seat stays holes to screw in the rear rack and the top clearance for the rear tire. my wife's fuji bike has the rear tire way higher than the holes on the seat stays.
Can i add any brand of rear rack to this bike? or should i look for something special for these cases?
It takes a little bit of bravery the first time, but you can easily apply some force and bend the front rack stays to reach down to the seatstay. The rack stays are designed to be bent, and doing so doesn't appreciably reduce the carrying capacity of the rack. The majority of the load is carried down the vertical portions of the rack to the dropouts, and the horizontal stays are purely there to prevent the rack from rotating forward or backwards - their effect on the load bearing capacity of the rack is negligible. Many constructeur bikes do away with those stays entirely, bolting the rack directly to the rear fender.
With racks, carrying capacity is far more important that brand or model. With the exception of racks that click into bags made by the same manufacturer (like Topeak), racks are mostly all the same. The exception to this is high-capacity touring racks, which get quite expensive and are overkill for anything but touring or hauling very heavy loads.
If you're concerned that you'll have to mount the rack on a bit of an angle, don't worry about that. However, if the rack stays are being blocked, then you have a problem that'll have to be solved with bending rack stays, and that'll reduce the effective carrying capacity and durability of the rack.
Another possible solution may be a seatpost mounted rack.They have a capacity of around 20lbs. Since they attach to the seatpost only there should be no clearance issues.
