Does someone know authors that wrote on the topic of small alliances? I'm writing a bachelor thesis on this topic and while there are tons of material on alliances in a classical term, I can't find anything on alliances between small states

I’d start with looking at the history of various small states.

You could even use currently extant ones that go back centuries. San Marino, Luxembourg, Andorra, Monaco spring to mind in Europe. A few not so old ones like Singapore also spring to mind.

Further than that, you could look at the dozens of tiny states that comprised the HRE. Many of them existed until C~ 1870.

That enough there for a dissertation on micro-state diplomatic dealings and conflict resolution since the European pre-renaissance era, methinks?

And plenty of people have written on these topics.