Are there any specific groups that are using those siddurim ??
Tefillas Shlomo Hashalom and Etz Chaim both are Sefard
Are they considered "Hasidim" Siddurim ?
I see that artscroll offers both.
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Nusach Sefard is the name for the many versions of the Hasidic siddur, so yes, these siddurim are Hasidic in nature. However, they do not fit into a particular Hasidic sect as a whole, although I am sure there are some shtiebels out there that use them. I own both Siddur Tefillas Shlomo and Siddur Eitz Chaim, and I can attest that they are exactly the same in nusach. The only thing that is different is that Siddur Eitz Chaim has an English translation facing the original Hebrew, and Siddur Tefillas Shlomo is all-Hebrew, including the instructions. Both are nice siddurim, I might add.
Most Hasidic sects have their own distinct version of Nusach Sefard, based on the way their Rebbe interpreted the writings of the Arizal and reconciled them to the original Nusach Ashkenaz. For instance, Levi Yitzchak of Berditchev made his own version of Nusach Sefard based on his interpretation of the Arizal, and this siddur is known as the Berditchever Siddur. Rabbi Joel Teitelbaum of Satmar himself only davened from such a siddur, and, truth be told, the Berditchever Siddur is a very popular siddur amongst Hasidim. Chabad, on the other hand, has Siddur Tehillat Hashem, a "Nusach Ari" siddur as interpreted by the Baal HaTanya. The Munkatcher Siddur is a Nusach Sefard siddur according to the minhagim of Munkatch.
Since every Hasidic sect has its own customs concerning the prayers, the industry of Nusach Sefard siddurim can oftentimes become confusing. Thus, ArtScroll and other publishers have attempted in (fairly) recent years to create a standardized, "universal" Nusach Sefard, Both of these siddurim in question you have asked about are this kind. "Universal," meaning, "not standing by one particular group of Hasidim."
Like I mentioned before, ArtScroll's Nusach Sefard is Hasidic in nature. I cannot name everything that makes it this way, but here a few things that make it stand out: Hodu before Baruch She'amar, Lechai Olamim, Patach Eliyahu, Yedid Nefesh, Ka'agavna after Kabbalat Shabbat instead of Bameh Madlikin, Kiddush (puts "ki hu yom" in brackets, etc.), the Amidah (Morid HaTal, etc.), Tachanun, and a bunch of Yehi Ratzon-s before blessings...Just to name a few. Although these things I have listed are indeed included in Edut HaMizrach siddurim of the Sephardi rite, I will mention that these siddurim are obviously Ashkenazi from these things: Birkat HaMazon, only ten lines in Adon Olam, Gott fun Avrohom before Havdalah, etc. Not to mention ArtScroll is an Ashkenazi publisher, and the transliterations in the siddurim are Ashkenazi in nature...
I would suggest you go with Siddur Kol Yaakov HaChadash, a "universal" Nusach Sefard siddur that a lot of Hasidim are quite fond of, or Siddur Tefillah Yesharah V'Keter Nehora, the Berditchever Siddur. You will find especially that the Berditchever Siddur seems to follow the writings of the Arizal better than these "universal" siddurim. Also, the Berditchever Siddur contains plenty of Hasidic sources and the typeset is usually in Ashuri letters!