1) The proper time for reciting selichos is after chatzos (halachic midnight). The prevalent custom (besides for the first night of reciting selichos) is to recite Selichos, before Shacharis prayers. 

The Mekubalim explain that one should not recite Selichos during the first half of the night at all (from nightfall until chatzos), and that one who does so causes great spiritual damage. (See Magen Avraham 565:5)

2) Based on the above teaching it would be inappropriate to recite selichos after Maariv (for example at 10:00 p.m.), which many shuls do. Rav Moshe Feinstein zt”l (Igros Moshe 2:105) was asked regarding a minyan which cannot recite selichos in the morning because the congregants need to go to work and they don’t have time. Rav Moshe writes that although prior to Chatzos is not the ideal time, there is no prohibition to say Selichos at such a time, and as a Hora’at Sha’ah (extreme case), one may be lenient. 

However, Rav Ovadyah Yosef zt”l (Chazon Ovadyah Yamim Nora’im p. 3-6) argues that there is a prohibition based on Kabbalah. And it is better not to say selichos at all than to recite it before chatzos. He concludes that if the congregation cannot recite selichos before shachris, they may say them with a minyan before  Mincha or they may recite it at work b’yichidus (without a minyan).

3) There are basically 2 issues when reciting Selichos without a minyan. The first is The Thirteen Attributes of Mercy. The verse of The Thirteen Attributes of Mercy may be said only in the presence of a minyan. According to the majority of the authorities if there is no minyan, it may be recited not in the form of the prayer, but in the manner of reading from the Torah (with the musical trop of the Torah reading). While some authorities, based upon the view of the Ari Z”l, maintain that it should not be recited in any form. (See Chazon Ovadia Yomim Noraim page 28 for a full discussion) For normative halacha, a rabbi should be consulted.

4) The second issue are the prayers that are Aramaic. The halacha is that one may not recite prayers in Aramaic without a minyan (M.B. 581:4). The reason is that when the Talmud discusses praying in Aramaic rather than Hebrew, it asserts that the angels do not understand Aramaic. Since we need the angels to carry our prayers on high, we should pray in a language that they understand. However, when praying with a minyan one recites Aramaic since the prayer of a minyan is received directly by Hashem, without the need of angels.