If by "one cube of tripple 7" you mean one pellet, sure that is safe to shoot. The gun is rated to 3 pellets of 777. Just make sure you always seat the bullet on the powder and you will be fine.
If you want an even cheaper plinking load, get some loose Pyrodex and some patched round balls (the tigher the patch the better) and shoot that with about 50 to 60 grains of powder. It's a cheap and fun load to shoot and can double as a small game load. My Mag Bolt likes the .490 ball with a .022 lubed patch and will hold a good group up to around 80 to 90 grains of powder. for lower powder levels a .018 patch should work too. Or you can get the .495 balls and use the .018 patch. Get some and go have some fun.