Andrew R Stewart 

I would wait on sealant filled tubes till you get tire mounting down pat. A foot long tube slash is nearly always from a portion of the tire bead not staying overlapped with the rim, the resulting hernia sees the tube balloon out through the gap and finally tear/explode.
There are a few common reasons why the tire bead might not be secure within the rim. The tube can get caught between the rim and tire bead, the valve base can prevent the bead from fully overlapping with the rim, the rim and/or tire might be made "off spec" and are not of matching diameters. All three reasons can be prevented from ending up as you did if the tire bead fit to the rim was done a few times during the inflation, catching the bulging off rim tire bead before the tube blew through the gap.
Besides checking the tire's seating a few times during the inflation one can try to see if the tube is caught under the tire bead before inflation by after mounting the tire/tube going around the wheel and prying the tire back away from one rim side and looking in the rim/tire to see if the tube is visible. The valve base needs to be pushed back up in the rim to allow the tire beads to slide down and fully overlap the rim's sides. Inflate a touch and check the fit as you did. Add 10 psi and recheck. Add 20 more and recheck. I would test inflate the tires about 10%-20% more than they will be used at (WRT the max allowed) to insure the tire is seated well and not creeping up off the rim. Andy