Adding the -in suffix is a “secondary derivative” or “taddhita” that makes an adjective. It causes the root vowel to take the vṛddhi grade, though e is the vṛddhi in Pāli. So it doesn’t change in this case - this is more of a feature of Sanskrit (cf jina > jaina).
There are many ways to make adjective in Pāli, as there are in English. Vādika is certainly a word meaning “one who speaks”.
But since theravādin is an adjective, you don’t need to add anything to it. Your problem is that if you want to use a Pāli plural then it is gendered and you’d have to specify which gender you mean.