Contrary to popular belief, the front brake is much safer to use than the rear. As you slow down, the weight of the bike is transfered forward. The traction on the front increases, and the rear decreases. It is then very easy for the rear wheel to break out from under the bike in a skid.
The front brake will not send you flying over the handlebars or crashing to the ground. It is the primary brake of a bicycle.
For slowing down, you need to "feather" the brakes. You can use the front or the rear. I often apply them alternately in quick succession. The trick is to modulate your braking power, so you apply just enough to do the job.
If your brakes are slipping on the rims, you may have contamination from chain lube. be careful that oil does not drip onto the rim when you apply it.
Wash your rims with detergent. Check your brake blocks for any stones or glass that may become embedded in the rubber, these will gouge out the rim surface. Clean the blocks with an old toothbrush if they are oily.