My slave cylinder was leaking

The best way to flush it without getting the old crud from the old fluid into your new slave cylinder is as follows;
Get a brake line, make sure the fitting is a matching pipe fitting as they are astandard and you will be able to find one that will thread in. They are 1/8" or 3/16". Cut it off long enougn to attach some soft clear polyvynil tubing to it. You can get it at Home Depot and while you are at it get some that will fit the bleeder nipple so you can use it to catch the system fluid later. Get some that fits snugly and then cable tie it so air can't get back in. Then run the hose over a point where you can go higher than the master cylinder and hang the hose. Get a catch can to catch dirty fluid. You will need an assistant to help you as the fluid has to stay above the intake holes or you will suck air into the system. Simply pump the pedal and fill the master cylinder as you do so. You may have to pull the pedal off the floor but that is okay as you don't have a closed system yet.
After fluid comes out clean attach the OEM hydraulic hose to the slave cylinder and then use the hose and catch can to catch fluid. Pump pedal and hold and release the or loosen the bleeder nipple. Then tighten it and then the pedal can be raised from the floor. Put some axle grease on the bleeder nipple fitting to keep air out and continue until no air bubbles are present in tube and fluid is clean and clutch disengages properly.

They do not thread into the pipe you bought or they are not the thread in type. Meaning that they either push on and clip or clip into place?

Good to hear. Come back anytime you need anything else.