A sadhaka is focused on his or her Ishta devata. What should be his/her attitude towards other devatas? What do our scriptures say in this regard?
From the Picchi tantra as found in Sarvollāsatantram 12th Ullhasa:
पिच्छिलायाम्-- गुराराज्ञां समादाय शुद्धान्त:करणो नर:। स्वेष्टदेवं स्तुवन् भाव्यमन्यदेवं न पूजयेत् ॥ १० ॥
According to Picchi Tantra, a Sadhaka with pure Conscience, must worship his Ishta following the order of his Guru. Never Worship any other God.
However it also says:
अन्यदेवं स्वेष्टरूपं रूपं ज्ञात्वा क्रियाञ्जरेत्। दर्पणेषु यथा बिम्बं तथान्यदेवरूपकम् ॥ १२ ॥
Similar to the image that forms into the mirrior, Sadhaka must see the other god as if it's his Ishta. He must imagine that all other gods are the images of his Ishta itself.
And as the comment says: Truth is one, men call it by various names.
एकदेवं बिना देवि नास्ति देवो महीतले। एकसूर्य बिना सूर्यो नास्तीह जगति यथा ॥ १३ ॥ बहुपात्रे स्थिते तोये बहुसूर्य यथा प्रिये। बहुभावे तथा देवं बहुरूपेषु दृश्यते ॥ १४ ॥
O devi, There are not many but there is only god on earth. There is only one sun in this world, where are the other suns? (13)
O devi, The Sun multiplies its number when looked in those all pots containing the water. The same way, due to the abundance god is not one in number but many in number. (14)
I am not translating the 11th verse, if you are intrested you can look into it:
तीर्थपर्यटनं देवपूजनं तर्पणादिकम् । वर्जयेत् परमेशानि गुरोरन्यं न भावयेत् ॥ ११ ॥