Does every Form of God worship Other Forms like Shakti worships Vishnu and Vishnu worships Shakti?
Is it true for Lord Shiva also?
What is the reason?
Yes, thats true.
This is common for each form of God .Let us take some examples:
Yataschaviraasit jagat sarvametat tathaavjaasano vishvago visvagopta/Tathendradayo devasangha manushyah sadaa tam Ganesham namamo bhajamah meaning: I always prostrate and worship Lord Ganesha, who is the origin of the universe and of Brahma (Abjaasana), Vishnu (Visvagopta), Indra and men (Stavakusumanjali, Swami Gambhirananda (ed), Udbodhan, page 154. The next sloka says Shiva is also born from Ganesha (Ibid page 155).
Ramaa-Shambhu-Brahmaa-Surapati-Ganesha-archiapada, meaning tge feet of Lord Jagannatha is worshipped by Lakshmi, Shiva, Brahmaa, Indra and Ganesha.(Ibid. page 268).
Vidhi-Vishnu-shioddhrita-paadayugam pranamaami Shivam Shivakalpatarum, meaning I prostrate to Shiva whose feet are held by Brahma and Vishnu (Ibid page 168-9).In this stotra Shiva is the source of all.
saamani yasya kiranaan prabhavaadihetum brahma-hara-atma-kamaalakshamchintyahetum meaning that Surya is the source of Brahma, Shiva and Vishnu (Stavananjali,R.K.Math Nagpur, page 84).
vadamaste kimvaa janani vayam uchchair jadadhiyah na dhata naapisho harirapi na te vetti paramaam meaning that Devi is beyond perception of Brahma, Vishnu and Shiva.(Stavakusumanjali, page 305).
This is allowed in our tradition. This is called Ishtanistha. Also known as 'Nahi-ninda-nyaaya' so far as I know.
After illumination, each saint realises that "Ekam vedam vibabhuba sarvam" The Same one is expressed as many.