"I notice that bugs are especially [bad] in my backyard ... [they] tend to feast on the leaves of the zinnia leaves if they are too short. The problem goes away once the zinnias grow tall."

That sounds like some pillbug damage I had a few years ago. They fed on the lower leaves of my short zinnias and didn't bother the taller zinnias. I mulched with play sand to create an environment they didn't like, and fed them a lot of pillbug/sowbug bait to reduce their numbers.

"I plan to put the Magellans in the front yard but I am not sure if their short height will make them a prey to bugs and other diseases."

Magellans, and other bedding zinnias like Dreamland and Peter Pan, look good in beds, but their lower plants do make them susceptible to soil splash during hard rains. The rain drops splash some soil up onto the lower leaves, and the soil can contain soil-borne organisms that cause foliage diseases. In my opinion, the good appearance of the bedding zinnias make it worthwhile to take steps to minimize the problem. A good mulch could reduce the soil splash. You could spray them to help prevent foliage diseases. I use a product called GreenCure®, which is safe and non-toxic and effective in preventing a variety of foliage diseases.

ZM

The slugs made my 'Magellans' look awful, while none of my taller zinnias (even my 8" tall 'Swizzles') had that problem. However, my 'Magellans' were shorter than they were supposed to be; large blooms right on the ground.