Although Vegetarianism and sentientism are not a central part of Sri Aurobindo's teachings, he is quoted (in the Internet, but I need to confirm this with actual references) as saying "Life is life – whether in a cat, or dog or man. There is no difference there between a cat or a man. The idea of difference is a human conception for man's own advantage." (this was probably said at the time when the Mother was keeping and got to know a number of cats personally; she often talks about these cats in her Collected Works). There is also another statement he makes (which I need to find and add here) regarding consideration and compassion to animals.
Sri Aurobindo wasn't vegetarian in the beginning. Even as late as the early 1920s, when he was practicing his supramental yoga in Pondicherry, he still ate meat, but was no longer the militant nonvegetarian he was when younger (Heehs - The Lives of Sri Aurobindo p.331). Then as his sadhana deepened he quit eating meat altogether and became a complete vegetarian. This was not for sentientist reasons, but because as the yoga progressed the desire for or any need to eat meat dropped away. (There is a reference in one of his writings to this, which I'll add when I have it)
In view of this, one wonders whether the original emphasis on vegetarianism in Indic mysticism was less for sentientist reasons (although it was in Jainism, and I have to say I side with the Jains in this, and the Pythagoreans would seem to have been influenced by Jains) than as a direct result of yogic siddhi. Or perhaps the two were equal factors. Ramana Maharshi who was a strong sentientist advised the eating "of sattwic food (and even that only) in small quantities"). Although vegetarian sentientist yogi was Ramalinga, a supramental diviniser anf predecessor of Sri Aurobindo and the Mother.
In any case, from the beginning of the ashram, a vegetarian lifestyle was the norm. According to a sympathetic account by Jatindranath Sen Gupta (circa 1927), in response to slander by the Catholic Church in Pondicherry at the time, life in the ashram was frugal, with one vegetarian meal a day (Peter Heehs, The Lives of Sri Aurobindo, p 359). The ascetic lifestyle may have been long relaxed during the War, when the ashram adopted many children and refugees, and hence it was impractical to enforce such rules. But even to this day the ashram serves only vegetarian food, and a vegetarian diet is standard among practitioners of Integral Yoga worldwide.
I have been told by a devotee I respect that the the Mother was always a vegetarian, even before she met Sri Aurobindo. But it seems (so someone else informed me recently) this was apparently for aesthetic reasons, not ethical ones (Mother's Agenda Vol 6, pp. 80-81 ), and when this was shown to be it was a big dissapointment, hence my preference for Ramana in this regard, despite my great respect and appreciation for Mirra in every other regard. In addition, elsewhere in the Agenda vol 10) Mirra speaks out strongly and with great sympathy against the Canadian clubbing of fur seal pups. As always, things are not simple!
In any case, she mentions somewhere in the Agenda that she and her first husband, Henri Morriset, a minor artist, travelled and lived a vegetarian lifestyle. I do not know whether Theon, her teacher in occultism, was vegetarian, although it would not be surprising as Theosophists were and are. Likewise Paul Ruchard, Mirra's second husband, and another student of Theon, was vegetarian and (I need to check the reference) sentientist.