by Sage Pearce-Higgins Wed Nov 22, 2017 7:54 am
In my experience, when students first understand parallelism they tend to overuse it. Verb tenses certainly don't have to be parallel; the example you give ("I am a loyal employee of my company and I will always be.") would be better if you omitted the second "I": I am a loyal employee of my company and will always be. The meaning is clear and the sentence is now fine.
So, yes, a verb with 'has' can be parallel to a verb with 'is': The company has experienced difficulties, but is now doing well. My preferred test for parallelism is to change the order of the elements and see if the grammar of the sentence works (although sometimes the meaning will change). In this example, you could write ;The company is now doing well, but has experienced difficulties.' and the sentence still works.
As for active and passive voice, the situation is more tricky. Generally it's not okay, but I wouldn't want to make a rigid rule here. The classic example is SC 765 from OG 2017.
Although it's good to consider questions such as the ones you posted above, I would encourage you to work mostly with real examples and not stray too far from GMAT material.