Yes, 兵法 can be read as heihou(へいほう) or hyouhou（ひょうほう). "Hyou" is more classic onyomi than "hei." Also, "heihou" tends to be used when it means "the war strategy; how to use soldiers effectively at a war." On the other hand, "hyouhou" tends to be used when it means "personal skill of martial arts." But this is not the absolute rule.

It's usual in japanese to find more than one pronuntation for the same kanji. Do you have the "onyomi" and the "kunyomi" pronuntation depending on there is the original chinese pronuntation or not and if it is a word acompannied with hiragana or another kanji. But that's not the only question, you have one or more onyomi or kunyomi readings. So, it depends on how are more used. As you say, 兵 can be readed hei or hyō. Then the word 兵法 can be readed "heihō" or "hyōhō". The meaning is the same, but I would pronounce "heihō", it's the more usual pronuntation nowadays. In hiragana it looks as へいほう.