I am under the belief that the Nirmaleys were originally KesDhari Brahmins.
In the past the main protectors of the Brahmins were the Khatree but they abandoned their religion and started to adopt the ways of the Mughals. Guru Sahib mentions in Asa Di Var
ਖਤ੍ਰੀਆ ਤ ਧਰਮੁ ਛੋਡਿਆ ਮਲੇਛ ਭਾਖਿਆ ਗਹੀ ॥
The K'shatriyas have abandoned their religion, and have adopted a foreign language.
For this reason Brahmins have always turned to the Khalsa for shelter even before Sri Guru Teg Bahadur Sahib sacrificed their life for the Kashmiri Brahmins. Shaheed Bhai Mati Das Ji, and Shaheed Bhai Sati Das were born as Brahmins before they took Amrit. Also Bhai Chuapa Singh Ji was also born as a Brahmin before taking Amrit.
The Turks knew that the Brahmins and their laws/customs needed to be completely wiped out for Islamic law to be in full effect. Thus the Brahimns had no where to turn to for help but the Khalsa. Many of them went to Anandpur Sahib for shelter.
Guru Sahib told Bhai Desa Singh Ji
ਗੋ ਬ੍ਰਹਮਨ ਕੀ ਰੱਖਿਆ ਕਰੈ l ਤੁਰਕਨ ਸੋਂ ਅਤਿ ਹਠ ਕਰਿ ਮਰੈ
ਭਾਈ ਦੇਸਾ ਸਿੱਘ ਜੀ
Protect the Brahmin and the Cow, and end the fanatical force of the Turks.
At Anandpur Sahib Dasmi Paatshah put these educated Brahmins to work. Eventually these Brahmins decided it was best to become Singhs so they started accepting the ways of Sikhi and Guru Ji gave them seva to do. Because these Brahmins were so use to their ancient rituals it was hard for them to give up some of their customs, and resort back to their trickery ways. Some of the customs that many could not give up were wearing the janue, wearing orange robes, and robbing the people. Guru Sahib made it clear that they must stay distinct and renounce all their clever ways if they want to be accepted as Sikh.
That KesDhari Sikh who wears a loincloth ( Brahmin robe) consider him an apostate.
ਜੋ ਸਿਖ ਕੇਸਧਾਰੀ ਲੱਗੋਟ, ਸੋ ਭੀ ਤਨਖਾਹੀਆ
and
ਕੇਸਧਾਰੀ ਬਗਲਬੱਦੀ ਜਾਮਾ ਪਾਏ ਸੋ ਤਨਖਾਹੀਆ
Those fake Kesdharis who wear orange robes regard them as apostates.
Even in the past the Pakhandi Brahmins would act like Muslims, adopt their ways when Hindus werent looking, and then they would act like Brahmins when the Muslims were not looking GUru Sahib says.
ਨਾਨਕ ਕੂੜੁ ਰਹਿਆ ਭਰਪੂਰਿ ॥
ਮਥੈ ਟਿਕਾ ਤੇੜਿ ਧੋਤੀ ਕਖਾਈ ॥
ਹਥਿ ਛੁਰੀ ਜਗਤ ਕਾਸਾਈ ॥
ਨੀਲ ਵਸਤ੍ਰ ਪਹਿਰਿ ਹੋਵਹਿ ਪਰਵਾਣੁ ॥
ਮਲੇਛ ਧਾਨੁ ਲੇ ਪੂਜਹਿ ਪੁਰਾਣੁ ॥
ਅਭਾਖਿਆ ਕਾ ਕੁਠਾ ਬਕਰਾ ਖਾਣਾ ॥
ਚਉਕੇ ਉਪਰਿ ਕਿਸੈ ਨ ਜਾਣਾ ॥
O Nanak, they are totally permeated with falsehood.
The sacred marks are on their foreheads, and the saffron loin-cloths are around their waists;
in their hands they hold the knives - they are the butchers of the world!
Wearing blue robes, they seek the approval of the Muslim rulers.
Accepting bread from the Muslim rulers, they still worship the Puraanas.
They eat the meat of the goats, killed after the Muslim prayers are read over them,
but they do not allow anyone else to enter their kitchen areas.
Guru States wanted to make sure that these Brahmins did not try to deceive the Singhs. So he warned us in numerous Rehatnamey. In rehatnama by Bhai Chaupa Singh Ji
In Bhai Nand LaLS rehatnama Guru Sahib states
ਅਤੇ ਜੋ ਬ੍ਰਾਹਮਨ ਤਮਾਕੂ ਪੀਵੈ ਅਤੇ ਮਾਸ ਖਾਵੈ ਤਿਸ ਪਾਸੋਂ ਜੱਞੂ ਪਾਵਨਾ ਮਹਾਂ ਹਤਿਆ ਹੈ
ਕਿਓੁ ਤਮਾਕੂ ਅਤੇ ਗਓੂ ਮਾਸ ਖਾਵੈ,ਤਿਸਾ ਬਰਾਬਰ ਹੈ
that those Brahmins who use tobacco and eat meat, but then put on the janeu ( Brahmin thread) are the biggest butchers. Consuming tobacco ( hukkah of the Muslims) is as sinful was eating a Cow.Guru Sahib then states that do not give to such Brahmins
ਤਿਸੁ ਬ੍ਰਾਹਮਨ ਨੂ ਦਾਨੁ ਨਾ ਦੇਣਾ
In Bhai Daya Singh Jis rehatnama Guru Sahib states the following
ਗੁਰੂ ਕੇ ਚੋਰ ਹੈਂ, ਲੱਗੋਟ ਧੋਤੀ ਧਵਲੀ ਕਤਿ ਬਾਂਧੇ ਔ ਕੇਸ ਨਗਨ ਰਖੈ, ਸੋ ਮਹਾਂ ਨਰਕ ਭੋਗੈ
Theifs of the Guru include those whom wear a loincloth ( orange robe), and keep their heard uncovered. They will end up in hell.A few lines down GUru Ji says
ਬਾਮ੍ਹ੍ਣ ਕੇਸ ਪਾਹੁਲ ਬਿਨਾ ਹੋਇ ਤਿਸ ਕੇ ਹਾਥ ਕਾ ਨ ਖਾਇ
That Bhaman who is without Kes and Amrit do not eat from such people.
Guru Sahib then states
ਦਨ ਨ ਦੇਇ ਜੋ, ਜੋ ਪਰਨਾਰੀ ਭੋਗੇ, ਫਾਰਸੀ ਪੜੈ੍, ਨਾ ਮੇਂ ਓੁਹਦਾ, ਨ ਵਹ ਮੇਰਾ
Do not give to those who enjoy multiple women ( other peoples wife), and read Farsi. I am not their Guru and they are not my Sikh.It is clear from this rehatnama that any KesDhari Brahmins who practices the Islamic ways is a kurehati ( apostate). In the same pangti Guru Sahib then explains not to eat and drink from the hands of such people.
So it seems from these rehatnamey that perhaps these early KesDhari Brahmins use to collect money for Guru Sahib, and they were on the path of becoming full fledged Sikh.
The following rehatnama indicates that Brahmins were taking Amrit
ਜੋ ਬ੍ਰਾਹਮਨ ਗੁਰੂ ਕਾ ਹੋਵੈ, ਤਾਂ ਓੁਸ ਦੀ ਸੇਵਾ ਟਹਲ ਕਰਨੀ ਦੂਣਾ ਨਫਾ
If a Brahmin becomes a Gursikh, then serve him and you will receive twice the benefit.Because these Brahmins as KesDhari were in Gurus company the Singhs would serve them. But Guru Ji was saying that by serving Singhs one gets twice the benefit then just serving Kesdhari.
I believe that term Nirmala was perhaps a nickname for KesDhari Brahmins. Maybe because these KesDhari were still stuck in their purity/cleansiness rituals. Even today many Nirmalas wear a pinkish/orange robe, and many of them believe in the Advaita Vedanta philosophy as Bhai Kulbir SIngh Ji mentioned. Plus if Guru Sahib sent 5 nirmalas to Benares to study the Vedas then they must have been Brahmins, because only Brahmins were allowed to study the Vedas.