As long as you stay logged-in to Telegram FOSS, while registering another Telegram App, you'll receive the authentication code inside it as a message.
Doing it that way, you'll not need Google Play Services - you'll otherwise, as far as I know, need Google Play Services with the official Telegram APKs.
After having set up the other Telegram App (ex. Nekogram), you can log out of the FOSS version, and delete it without issues.