Ishwara/God is not limited to only a few forms. Ishwara can take any form. For worship, the form of Ishwara does not matter. What matters is the correct understanding of the tattva (nature) of Ishwara. The tattva of Ishwara comes from scriptures.
Ishwara can be worshipped in any form.
yē yathā māṅ prapadyantē tāṅstathaiva bhajāmyaham.
mama vartmānuvartantē manuṣyāḥ pārtha sarvaśaḥ৷৷ Bhagavad Gita 4.11৷৷
Whoever resorts to Me in any manner, in the same manner do I favour them; men experience Me alone in different ways, O Arjuna.
Ishwara has innumerable forms.
śrī bhagavānuvāca,
paśya mē pārtha rūpāṇi śataśō.tha sahasraśaḥ.
nānāvidhāni divyāni nānāvarṇākṛtīni ca৷৷ Bhagavad Gita 11.5৷৷
11.5 The Blessed Lord said - O son of Prtha, behold My forms in (their) hundreds and in thousands, of different kinds, celestial, and of various colours and shapes.
Ishwara can be formless (unmanifest) too.
mayā tatamidaṅ sarvaṅ jagadavyaktamūrtinā Bhagavad Gita 9.4.1
This entire universe is pervaded by Me, in an unmanifest form
Ishwara assumes all forms.
rūpaṃ rūpaṃ pratirūpo babhūva, tadasya rūpaṃ praticakṣaṇāya |
indro māyābhiḥ pururūpa īyate...Brihadaranyaka upanishad 2.5.19
‘(The Lord) transformed Himself in accordance with each form; that form of His was for the sake of making Him known. The Lord on account of Māyā is perceived as manifold..