Sorry, I don't know what you mean by "pojd'te" being special and irregular. It is possible to say "pojd' sem" ("come here"), but is it incorrect to say "nepojd' sem" ("don't come here")? Is it necessary to say "nechod' sem"? Thanks again.
Your Duolingo settings and the level you are on decide, not the exercise. Besides, installing keyboards is easier on the phone then anywhere else. I regularly and easily switch between Russian, Czech, English, German, Latin, Classical Greek, Old Cyrillic, Glagolitic.