writing katakana words with hiragana?

No, not OK. It looks awkward. But if you don't learn katakana yet, you can use hiragana as substitute.

Although most people who know Japanese would be able to read that, it isn't grammatically correct would cause some difficulty in understanding. It is always best to write katakana words(foreign words) in katakana and Japanese words in either kanji or hiragana.

Not unless you're trying to be edgy. You would not see these words written as such besides perhaps in a rock song's titles and/or lyrics.

あるみにうむ is good.

まいくろふおん is wrong.

マイクロフォン is まいくろふぉん

（おぉおぉ）

Normaly, however, those words are never written with Hiragana.

Because loan words is written with Katakana.