Q] Can a Bālā upāsaka worship the śrīcakra?
Anyone can do anything. The question really is what is right according to śāstra dr̥ṣti? Though śrīcakra represents a sarva-samaṣti śakti cakra, Bhagavatī Bālā is one of the aṅga śaktis of Lalitā in spite of the abheda that exists between Bālā, Bhairavī, and Sundarī. It is said, bālalīlā viśiṣṭatvādbālēti – Lalitā on account of her bālālīlā viśēṣa, is called Bālā. But apart from mantra and svarūpa, the difference can also be seen between Lalitā and Bālā in terms of adhiṣṭhāna, which is very important for śākta saguṇōpāsanā. The adhiṣṭhāna of Bālā is in the Svādhiśṭhāna chakra, which is the seat of letters from baṁ to laṁ, evident in her very name.
Moreover, a specific yantra for Bālā is described in various Tantras and this Yantra needs to be worshipped by upāsakas of Bālā Paramēśvarī.
Acharya Gauḍapāda describes the yantra thus:
tadēva patravasudvayayuktamanusvaraṁ kumārīsadanam ||
Shaṅkarāraṇya describes the resulting Bālā chakra thus:
bindu trikōṇa ṣaṭkōṇa vr̥ttāntaraṣṭadalapadma vr̥ttabahiraṣṭadalapadma caturaśrātmakaṁ bālā cakraṁ bhavati ||
An alternate Saptāvaraṇa yantra is described in Traipura Siddhānta:
tatō yantraṁ likhēddhīmān bindumādau likēhnnaraḥ | trikōṇaṁ vilikhēt paścāt ṣaṭkōṇaṁ vilikhēttataḥ | dalāṣṭakaṁ likētpaścāt tadbahirvr̥ttamālikhēt | caturaśraṁ likhēt paścāt caturdvārasamanvitam | maṇḍalaṁ vilikēht paścādyantramētadudīritam | saptamāvaraṇōpētāṁ śrībālāṁ praṇamāmyaham ||
As the upāsanā of Bālā through this Yantra is taught as a preparatory practice to the worship of Lalitā in śrīcakra by various upāsana śastras of śrīkula, the yuktatva of worship of Bālā in her own yantra becomes evident. This is also the case when the mukhya dēvatā worshipped is Bālā and not as a parivāra dēvatā of Lalitā.
To summarize, there can be two cases:
a. The person is a upāsaka of Bālā alone: In this case, it is quite evident that he needs to worship the Bālā yantra. The same śāstras which describe śrīyantra in relation to Lalitā, also speak of a separate Bālā yantra. If the upāsaka of Bālā could simply worship śrīyantra, Bālā yantra would more or less be redundant. Trikūṭā Rahasya states Pañcadaśī as the least requirement for pravēśa into the Navāvaraṇa yantra of Sundarī. Bālā (tryakṣarī or śadakṣarī) is a pūrvāṅga of Lalitā like Mahāgaṇapati, worshipped prior to undertaking the upāsanā of Lalitā.
b. The person is a upāsaka of Sundarī: If this is the case, in both nitya and naimittika pakṣas, he would need to worship Sundarī and not Bālā. This is based on the assumption that he received initiation into the Mahāvidyā of Sundarī after the prescribed upāsanā of Bālā, generally a pre-requisite in many sampradāyas. The lack of need for worshipping pūrvāṅga dēvatās after initiation into the mukhyavidyā is discussed at length twice by Rāmēśvara. In the case of kāmya pakṣa, if one desires to worship Bālā for a specific purpose, he can then make use of the sarva-śakti-samaṣṭitva clause of the śrīcakra and worship Bālā within the śrīcakra, based on the pramāṇa from Tantrarāja or Kādimata.