Why are householders advised not to venerate fierce forms of Kali? I was told by those who follow the shakti tradition that householders should not honor the angry forms of Kali. Why is this?
That is not true. There are no such fierce forms of Goddess KAli.
She is fierce apparently but at the same time endowed with motherly affection. So, she can perfectly worshiped by everyone irrespective of whether one is a householder or a sannyAsi. She is a perfect deity for the householders in fact.
I have done a preliminary study of the Dasha MahAvidyAs and still doing so. She is the first MahAvidya in the group. And, she does not have Murtibhedas (that is different forms of the one principle Deity).
So, there are no such classifications such as- pleasant forms KAli, fierce forms of KAli etc.
The second MahAvidyA, Goddess TArA, however has Murtibhedas. But all her forms are Bhoga-Moksha pradA and can be worshiped by all.
TArA chogrA mahogrAcha vajrA nilA sarswati | KAmeswari bhadrakAli ityashtau tArini smritAh ||
TArA, Ugra, MahAugra, Vajra, NilA, Saraswati, KAmeswari, BhadrakAli; These are the eight Murtibhedas of Goddess TArA.
Now, apparently Goddess KAli will seem as terrifying but that will be only for persons who are at their lowest level of spiritual evolution. The scriptures say that she is known as MahAkAlki since she devours even MahAkAla, who himself devours everything at the time of dissolution.
Tava rupam mahAkalo jagat sanhAra kArakaha | KalanAt sarva bhutAnAm mahAkAlah prakirtitAh || MahAkAlasya kalanAt tvamAdyA kAlikA parA ||..
[Lord Sadashiva says to Sri AdyA] Your form, MahAkAla, devours everything at the time of dissolution of the universe (samhAra). Since he devours (kalan) everything and every being during samhAra he is known as MahAkAla. And, since you devour even MahAkala, you are known as the primordial (AdyA) and supreme KAlikA.
MahAnirvAna Tantram 4-30,31.
So, because of this terrible samhAra murti, she is bound to terrify the weak minded. But for the higher level sadhAkas, she is appears as the affectionate Mother.
Her Dhyana Sloka that's why mentions her both as terrible (viz-KAli karAla VadanA) and at the same time as the affectionate Mother ( viz-Hasnmukhim, SmerAnana saroruhAm; having a smiling, affectionate and pleasant face).
Now, coming to the overwhelming importance of KAli worship in Kali Yuga.
ShadAmneshu deveshi bhuyashyah sashti devatAh | TAsu kAshchit kritayuge tretAyAm kAshchidiritAh || DwApare phaladAh kAshchit kalau kAshchit phalapradAh | Chaturyugeshu phaladA dasha vidyA mayeritAh || TAsu tisro vishishyante kAli tArA cha sundari | Tisrishvapi shive tAsu kalau kAli vishishyate ||
In the six amnAyas (Simply put, AmnAya is the original source of all Tantras spoken by Lord Shiva, from his six faces came six AmnAyas which is the source of all Tantras and Agamas) there are numerous deities Among them some gives fruits in Satya Yuga while some others give fruits in Kali Yuga. The Deities who give fruits in all the four Yugas are the Dasha MahAvidyAs. Among the MahAvidyA deities, KAli, TArA and Tripura Sundari are specially significant. And among these three, KAli is particularly effective in Kali Yuga.
MahAkAla Samhita's Anusmriti prakalpa.
So, needless to say, such an important deity can not have such restrictions over its worship.
In my opinion, your friends must have talked about another MahAvidya, the 6th one, Goddess ChinnamastA or Prachanda ChandikA.
The scriptures do say that there is no difference between KAli and ChinnamastA, so your ShAkta friends must have meant ChinnamastA as the "fierce form of KAli".
YathA chinnA tathA kAli tathaiva sundariparA | Tathaiva tArA samdishta chaturanAm nahi bhinnatA ||
Who is Chinnamsta, is KAli, she is [Tripura] Sundari and she is TArA as well. There is no difference between the four Deities
Shakti Samgama Tantra 4-51.
Now, as far as Goddess ChinnamastA is concerned, there are certainly some serious apprehensions. The scriptures clearly say that she is an extremely fierce deity and that there is no deity who is more fierce than her.
NAtah paratarA kAchidugrA devi bhavisyati | TasmAd sattairmurjaina grAhyeyam kathanchana || Siddhirva mrityurapi vA dwayorekataram bhavet || ...
There is no deity which is more fierce than ChinnamastA. That's why faint hearted and weak person should not receive her mantra. In her sAdhana either Siddhi is achieved or Mrityu (death).
ViswasAra Tantram verse quoted in PurashcharjAnava's Taranga 9.
Yet another verse depicting the terrible nature of Goddess is the following one:
PrachandachandikAmevadhyAtvA yastu na pujayet | Sadyastasya ShirashchitvA devi pivati shonitam ||
One who worships Chinnamasta or PrachandachndikA without her meditative verse, Devi cuts his head off.
The significance of this verse is that, while other deities can still be worshiped without employing their DhyAna Slokas, ChinnamastA simply can not be. Such is her fierce nature. So, a ritualistic worship of Goddess ChinnamstA is not safe to practise in homes on ones own without the guidance of an able Guru.
And, as far as Devi's DhyAna Slokas are concerned, there are indeed many of them. And more importantly, there are separate DhyAnas for Yatis (sannyAsi) and for Grihastas (householders).
The DhyAna for sannyAsis starts like this:
SvanAbhounirajam dhyAyet bhanu mandala sannibham | Yonichakra samAyuktam gunatritraya samgitam || Tatra madhye mahAdevim chinnamastAm smared yatihi ||
And, the DhyAna for the householders starts like this:
PratyAlirapadAm sadiava dadhatim chinnam sirah katrikAm....
And, as far as goddess MahAkAli is concerned, there are no such restrictions.
In Bengal, plenty of homes even have MahAkAli Temples within the premises of their homes.