What if a devotee is attached only with the Radha Krishna form, not other forms or avatars?

It is natural.

SRI Vaishnavas never speak about the lilas of Radha Krishna and they follow the worship of Lord Narayana and Vishnu.

Gaudiya Vaishnavas prefer Radha Krishna worship. They also have the worship of Sri Caitanya Maha Prabhu and Sri Nityanandha (Gaur Nitai), Baby Krishna, Lord Jagannatha, etc.

You may be aware that Hanumanji, who is the best example for devotion as a servant of the Lord, does not prefer to see even the form of Krishna and he wanted to see only Lord Rama and Sita devi.  He did not allow any deviations from his master Rama and Sita devi.  He lived only for them.

In the stories of puranas, we have read that once Hanuman was called by Krishna. When the messengers called Hanuman that Krishna calls him, he did not go though Krishna called him, saying that he will see only his master Rama.

Similarly, Gopis wanted to see only the Krishna form. They had Prema on Krishna. Prema on Rama is not possible.

Those who develop prema on Krishna would like to worship Radha Krishna form.

See, Radha Krishna worship is the topmost form of worship in Krishna Bakthi. We need to be as pure as possible in thoughus, actions and even body to worship Krishna.

Even in the new temples of ISKCON, they do not instal Radha Krishna deities because they need many hands to follow perfect rituals. They start with Gaur Nitai deities or Lord Jagannatha deities because they forgive any small omissions or delays in the rituals and the level of purity is not that much required to Radha Krishna worship.  Once the temple grows with sufficient hands to serve, they start Radha Krishna worship.

Narasimha deva worship should be still more serious.

So, only developed temples follow Radha Krishna and Narasimha worships.

However, some devotees are attracted with the worship of Radha Krishna form only.  This is natural. So, they can have the worship of Radha Krishna as their primary worship and also keep Gaur Nitai as additional worship.

And, since Radha krishna worship requires more care, they can use Pictures of Radha Krishna or the small deities of the height less than 3 inches.

This will serve their desire to worship Radha Krishna form.

I like to encourage deity worship in temples and centers only because they can maintain the required purity standards. So, the devotees may use pictures as big as possible with no size restrictions.

In case they are particular about worshiping the deities, let them use small deities of less than 3 inches height along with a bog picture of their favourite form of the Lord – either Gaur Nitai or Jagannatha or Radha Krishna.

Anyone of any basic standard of chanting and following four regulative principles, can worship the picture of Radha Krishna or any other favourite forms.

So, my solution to this psychological setting of prefering a particular form of Krishna, they can follow the primry worship of their favourite form, say, Radha Krishna as big pictures and alongside the normal sized pictures of other forms such as gaur Nitai or Jagannatha or Krishna-Balarama or Baby Krishna (Bal Gopal), etc.