You can start doing it right now, you need to apply, to access Testnet:
https://renprotocol.typeform.com/to/kARAnf

We very much encourage dev teams to do this now via MVPs and start experimenting!
With these docs:
1) RenVM Overview Documentation: https://docs.renproject.io/ren
2) RenVM Developer Documentation: https://docs.renproject.io/developers
3) RenVM SDK: https://www.npmjs.com/package/@renproject/ren
4) Example for Devs: https://github.com/renproject/uniswap

ZEC - may not be as popular, but this is your subjective opinion.
It is important to understand that efficiency is not measured by the number of connected blockchains.
Why do? Because if you have built your own Dapps based on RenVM - you can connect any blockchain to your DApps yourself. At the same time, you can configure the exchange of liquidity with other Dapps, according to pre-established rules. To do this, you do not need to contact the development REN team. Just as you do not need to contact the developers of a third-party blockchain that you want to connect. You ask such questions (about the number of supported blockchains) because you are used to thinking about interoperability as a standard to be adopted. But other blockchains should not take REN in as standard, so REN could work with them.

Going back to Zcash -  it's a delightful project with very hard cryptography. To establish the interoperability between ZCash and Ethereum, preserving the built-in propertis privacy of transactions, which have zcash - is difficult, but really.


*Perhaps your next question will be about the speed and performance of REN you can safely ask, I'm happy to answer you in detail.  But I will say in advance that speed is important, but the answer will be similar to the answer "about the number of supported blockchains":
It does not matter how many transactions per second - it's important to have enough Tps to keep up with the speed of a compatible blockchain