こととて is a stiff, bookish way to say 'because' or 'given that.' It often appears in official notices, business correspondence, or historical narratives. You attach it to the plain past or non-past form, but in practice it's most often used with past tense or with nouns to state an existing situation. The nuance is that the reason justifies something, like an excuse or a necessary condition. For example, 初めてのこととて (since it's the first time) sounds like you're excusing possible mistakes. It's not something you'd say to a friend—it would sound like you're quoting a legal document. Stick to written contexts or formal speeches.