There should be no problem mixing simplefit with another program, but keep in mind while mixing two different programs you will lose weight and gain strength, but you won't excel at either. What I mean by that is by dividing your focus between two different workout strategies you won't be able to dedicate yourself 100% to either; so it will take a lot longer to hit L7 on simplefit than if you were doing just simplefit on its own and, similarly, it will talk you longer to increase your lifts on a weight program than if you were doing just weights. But it sounds like you're just looking for general fitness so there should be no issue as long as you schedule in adequate rest days for recovery.
I'm currently mixing simplefit, crossfit, and stronglifts to prepare for a spartan race so that I'll have a good mix of strength and cardiovascular endurance, but I'm noticing that the poundages on my lifts has suffered while my cardio has increased slightly. So it comes down to knowing what your goals are and modifying your training to meet those goals. And there's nothing saying that you can't change your goals 6 months down the road and change your program to match. Just don't fall into the trap of changing programs every couple of weeks. You should give each program at least 4-6 months.
On a side note, if you're going to pursue a weight program, go with the free weights and stay away from the machines. Machines are fine for the weekend gym warrior, but if you want to build muscle and strength, free weights are the way to go. Harder to learn in the beginning, but worth it.